Preparing / pre-cooking hash browns / home fries
This answer is for sliced or diced home fries, such as this type:



Shredded style hash browns are much more difficult to pre-cook at home, since the shreds are fragile and go from under cooked to over cooked quickly.

Note that restaurants tend to simply have home fries on the griddle, in a warm section, essentially fully cooked.  This is why they tend to be more cooked at the end of breakfast service than the beginning in many restaurants.

For home service, there are several effective techniques for making home fries cook more rapidly, or at least more conveniently.

Par-cook the potatoes

The easiest for pre-preparation is simply to par-cook the potatoes.  In either case, the potatoes should be peeled (optional), and diced or sliced.

Cook them until they are just slightly under-done, but begining to be fork tender.  This will depend on your type of potato.  If you are using a waxy potato (such as a Red Bliss), you can cook them until they are fork tender as they hold their shape quite well; if you are using a starchy potato such as a Russet or Idaho, you want them just slightly resisting in the center.

After they are par-cooked, you may optionally refrigerate or even freeze them before finish cooking.

There are several methods which are effective for the par-cook, depending on your volume:


Microwave, suitable for a couple of potatoes.  Put plastic wrap or a loosely sealed lid on the container so they steam as well, and stir them fairly frequently to be evenly cooked.
Steaming.  Probably more work that it is worth, but very effective.
Simmering/boiling.   Start the potatoes in cold water.  Depending on the size of your dice or slice, they may be done about the time the water begins to boil; in any case, simmer them until done as described above.  Drain well, or even dry with towels.


To finish cooking them, fry them on a hot griddle with generous butter (or other fat, to taste), salt, pepper, and other additions that you desire such as onions, peppers and so on until they are brown on all sides, and somewhat crispy.  This will take about 5 minutes per side depending on your heat.

Hybrid wet-dry

This is the method I personally use the most, as the results can be outstanding, although you need experience to judge exactly how they should come out.   It is also only for reducing the cooking time, more than for pre-staging some of the preparation.


Dice or slice, and optionally peel your potatoes
In a wide pot where the potatoes will form a shallow layer (preferably non-stick, or they will sick a little and form a lot of fond, which will make it harder to get them crispy), add the potatoes and water to barely cover.  You will learn how much water over time for your cook top (hob), dice or slice size, and flame level.

You can also add butter, salt, pepper and such at this stage.
Cook the potatoes on high for about 10 minutes or until the water is gone.  Ideally, if guaged correctly, this will leave you potatoes at the "almost done" stage described in the par-cooking method.
Add fast cooking vegetables such as onions or peppers.
Continue cooking on medium high heat, stirring once or twice, until the potatoes are nicely golden brown and delicious.


Oven hybrid method

This method can be used for pre-preparation.  The finish cooking takes longer, but is hands off.


Prepare potatoes as usual, and pan fry them until browned.  They will still be essentially raw inside.  That is okay.
At this point, you can reserve and pause.
To final cook, place (with salt, pepper, slightly pre-cooked onions or peppers, and other enhancements) in a moderate oven to heat through and re-crisp.    Depending on temperature, this can take 20-30 minutes.


This method will not give perfectly crispy potatoes like the first two methods, but is suitable for having a large quantity, as for a brunch party, ready for service at the same time.