Cauliflower quinoa burgers with mushrooms and onions

small head cauliflower, chopped into small florets

olive oil, divided

salt and pepper

dry quinoa

vegetable stock

minced fresh parsley

fresh thyme leaves

panko breadcrumbs

large egg

butter

medium sweet onion, peeled and sliced into 3/4" slices

medium cremini mushrooms, sliced

medium cloves garlic, minced

large slices aged havarti or swiss cheese

burger buns (toasted if desired)

arugula and mayonnaise for serving (optional)