Pizza, Pizza, Deep Dish Pizza!
JavaScript (ES6), 136 bytes



f=
n=&gt;"/\\"+(r=s=&gt;s[0][0].repeat(n-1)+s)`_`+r`_`+r`___/\\
`+r` `.replace(/ /g,"$`\\ \\$`   $'$'$'/ /\n")+r` \\ \\`+r`_/ /
`+r`  \\`+r`___/`
&lt;input type=number min=1 oninput=o.textContent=f(this.value)&gt;&lt;pre id=o&gt;




The whole pizza is very repetitious so the r function (designed as a tagged template literal) repeats the first character of its input n times. This handles the top and bottom two lines of the pizza. The middle is repeated by replacing a string of blanks; the $` and $' subsitutions automatically correspond to increasing and decreasing numbers of blanks thus positioning the \ \ and / / appropriately.