Clean mushrooms and remove the stems. Dice the stems and keep them aside in a bowl.
Slice vegan sausages ("hotdogs") and asparagus.
Cut spinach leaves.
Turn the oven to 420F (215C).
Cut cauliflower head into florets.
Fill the mushroom caps with diced vegetables and sausage and place them on the baking sheet.
Place cauliflower florets on the baking sheet, and drizzle mushrooms and cauliflower with vegetable oil. Add salt and pepper.
Bake in the oven for 15 - 20 minutes and serve.