Clean the mushrooms and carefully break out the stems. Chop mushroom stems finely. Heat oil in a pan and fry mushrooms for about 5 minutes, turning them over. Peel onion and cut into fine cubes.
Clean, wash and dice the peppers. Coarsely chop the olives. Thyme wash, shake dry, pluck leaves from the stalks except for something to garnish. Knead minced meat, onion, mushroom stems, mustard, paprika, olives, breadcrumbs and milk.
Season with salt, pepper and thyme. Fill mushrooms with minced meat mixture. Cook in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 20-30 minutes. Arrange and garnish with thyme.