Preheat oven to 300ºF and line a rimmed baking sheet with parchment paper.

Arrange the sliced tomatoes on baking sheet in a single layer, season with salt and pepper, and drizzle with olive oil. Roast for 30 minutes, then scatter the garlic over the tomatoes and roast an additional 15 minutes. Remove from the oven, position a rack about 4 inches below heating element, and set temperature to broil.

Drain 2 tablespoons of the olive oil off the roasted tomatoes into a Dutch oven (reserve remaining oil for another use, such as salad dressing) and place over medium-high heat.

Add the sausage and onion and cook, stirring and breaking up large pieces with a spatula, until sausage is browned, 6-8 minutes. Add the minced garlic and tomato paste and cook, stirring, until garlic is fragrant and tomato paste darkens, 2-3 minutes. Add the chicken stock, roasted tomatoes and garlic, and tortellini, then season with salt and pepper. Bring to a boil, reduce heat to maintain a simmer, and cook, stirring occasionally, until pasta is almost done, 12-15 minutes. Stir in the halved grape tomatoes and cook, stirring, until softened, about 3 minutes, then stir in Parmesan and fresh mozzarella.  Taste and adjust seasoning. Transfer to an oven-proof serving dish and broil until lightly browned and serve hot, garnished with basil and passing hot pepper flakes at the table.