Cut off tops of the tomatoes and scoop out the insides. Clean the mushrooms and cut off the stems. Peel the garlic.
Place the mushrooms and garlic in the bowl fitted with the ultrablade knife and mix at speed 12 for 30 seconds. Replace the ultrablade knife with the mixer and add olive oil, scraping down the sides of the bowl if necessary. Launch the P1 simmer program at 270°F for 10 minutes.
At the end of the program, remove the mixer. Add ground sausage and breadcrumbs. Season with salt and pepper. Mix the stuffing with a spatula.
Place the tomatoes in an oven dish and fill with the stuffing. Add the tops and bake for 40 minutes or until sausage is completely cooked.