Preheat oven to 425°.
Wash potatoes, cauliflower, and broccolini (or whatever vegetables you choose to make).
Cut larger potatoes in half or quarters, or leave whole if they are very small.
Toss potatoes in a big bowl with about 3 T. olive oil, and lots of salt and pepper.
Spread potatoes out in a single layer on a baking sheet, and roast while you prep the rest of the veggies.
Cut cauliflower into bite size pieces.
Cut broccolini into bite size pieces.
Wipe out the bowl for the potatoes with a paper towel or flour sack towel, and use it to toss the cauliflower with more olive oil, salt, and pepper.
Spread cauliflower out in a single layer on a baking sheet.
Wipe out the bowl again, and repeat with broccolini.
Put cauliflower and broccolini in the oven with the potatoes.
Roast potatoes for about 30 minutes total, and the cauliflower and broccolini about 20 minutes.
Let all vegetables cool, then put them into serving bowls or dishes. These can be done the day ahead, put into ziplock bags, and refrigerated until the day of the party if you wish. If you make them ahead, pull them out of the refrigerator early, so they can come up in temp a bit before dinner.
Wash the whole mushrooms. If they are too big, cut them in half.
Sauté mushrooms in a large skillet with half a cube butter.
After about five minutes, add the minced garlic and 1/4 cup chopped parsley.
Sauté until mushrooms are cooked. Put them into a strainer over a bowl to allow the juices to drip off.