Drizzle the sprouts with olive oil, place them cut side down on a sheet pan, and then bake them in a preheated 400 degree oven for 20-30 minutes, until browned and soft.
Meanwhile, whisk together mustard and mayonnaise. When the sprouts are done, toss with the mustard-mayo mix, and season to taste with salt and pepper.
Garnish with chopped herbs if desired.