What does it say in gigantic characters on this building labelled "Treasure Trove of Royal Chinese Cuisine"? (Characters identified: 中華皇家御膳之寶)
Using an excessive number of clericalified variants (隸定字), I would say that the closest characters are

蕐皇家御膳*寶

In the Shuowen Jiezi lesser seal standard, the closest characters are

中家御膳*寶

*Actually, the older structure of 「善」 is 「」 (⿱羊言), and that is what the photo and Shuowen seal form both are

In a more modern orthography,

中華皇家御膳之寶
Treasure Trove (寶) of (之) Royal (皇家御) Chinese (中華) Cuisine (膳)

There is no character where the doorway is, so there are eight characters in total. The rest of this answer will focus on the more difficult to identify characters.

中
商甲甲398合集32500商金中且觶集成6213西周金七年趞曹鼎集成2783秦簡秦197睡虎地秦簡漢隸唐公房碑　今楷　
「中」 depicts a military flag with flowing streamers. This was later extended to mean authority (中央), then centre, middle.
The streamers stopped being written as part of the character very early on, but sometimes they were added back in decorative scripts as horizontal lines protruding from 「中」 (, , etc.)

In contrast, 「乍」 looked incredibly different in early scripts.


華
西周金不栺方鼎集成2735秦簡編年紀34睡虎地秦簡東漢隸禮器碑　今楷　
「華」 (Baxter-Sagart OC: /*qʷʰˤra/) originally depicted a flower, now written as 「花」. The petals of the flower would be stroke-transcribed as 「」 in regular script (e.g. in the clericalified Shuowen lesser seal variant 「」). The bottom of the stalk was drawn in the shape of 「于」, which doubled as a phonetic hint (/*ɢʷ(r)a/).
Semantic 「艸・艹」 (grass) was added on later (see second form above).
「華」 was also overwhelmingly used for the meaning flourishing &gt; magnificent, splendid (/*N-qʷʰˤra/) very early on via semantic extension from flower. The character 「花」 was invented later to represent the original sense of 「華」.

之
You're basically looking at a branch evolution here:

Series A:
商甲鐵16.1合集11654春秋金秦公簋集成4315春秋金姑馮同集成424篆之部說文解字隸定　

Series B:
商甲鐵16.1合集11654戰國・晉金兆域圖銅版集成10478戰國・楚簡12包山楚簡戰國・楚簡129包山楚簡秦簡法141睡虎地秦簡今楷　


「之」 (/*tə/) originally depicted a foot 「止」 pointing away from an abstract place represented as a horizontal line 「一」, indicating the meaning to go. 「止」 (/*təʔ/) simultaneously hinted at the sound.


「之」 (to go) and 「止」 (foot) are cognate.
Foot is now written using the derivative 「趾」.



The shape 「」 is what you find as a character component in other characters, e.g. in 「寺」 or 「志」. Normally, it is further corrupted into 「土」 or 「士」 in the modern form.

「之」 is the stand-alone character.



寶
商金作父癸集成5360西周金父乙簋集成3862西周金鄭虢仲簋集成4025春秋金國差集成10361戰國・楚簡221包山楚簡今楷　
「寶」 (Baxter-Sagart OC: /*pˤuʔ/, treasure/valuables) actually had about a million variants, mainly because Chinese characters weren't standardised back then and about 90% of bronzeware had the character 「寶」 inscribed somewhere. These variants do share about half their components with 「寶」, however.
The structure which survives today contains semantic 「宀」 (building), semantic 「玉」 (jade), semantic 「貝」 (cowrie shell &gt; money), and doubly semantic and phonetic 「缶」 (/*p(r)uʔ/, jar). The whole character indicates valuables (玉, 貝, 缶) stored inside a building (宀). If you see two or three components out of 「宀」, 「玉」, 「貝」, 「缶」 in a character, you're very likely looking at 「寶」.
The non-standardisation back then affects not only the components' presence, but the location of the components. On a component-transcription basis, the forms displayed here (listed from left to right) are, using ideographic description sequences,

寶 (same structure as the modern form)
⿱宀⿰⿱玉貝缶
⿱宀⿰缶⿱玉貝
Same as 2.
Same as 1.


「王」 and 「玉」 had different histories, but converged in shape later. As a character component, the shape 「王」 almost always represents 「玉」 (e.g. 弄, 玻). The times where the shape 「王」 actually represents 「王」 is mostly on the right hand side or the bottom of the character (汪, 皇), and these characters are pretty rare.


References:

季旭昇《說文新證》
小學堂
國學大師

郭沫若《甲骨文合集》
中國社會科學院考古研究所《殷周金文集成》


漢字源流
中國哲學書電子化計劃

宋・徐鉉等奉敕校定　漢・許慎《說文解字》