Set your oven to 375°F (190°C).
In a 9x13-inch baking dish, place the Boursin cheese right in the center. Surround it with sliced onions, tomatoes, and whole garlic cloves.
Drizzle everything with olive oil, then sprinkle with salt, black pepper, and red pepper flakes to taste.
Bake uncovered for 25 minutes, or until the tomatoes are bursting and the cheese is soft and melty.
While the veggies are roasting, bring a large pot of water to a boil. Salt the water generously, then cook your pasta according to package directions.
Reserve ½ cup of the pasta water before draining—this can help loosen your sauce later if needed.
Once the tomato mixture is out of the oven, use an immersion blender right in the baking dish to blend everything into a smooth, creamy sauce. (Careful—it's hot!) If you don't have an immersion blender carefully transfer to a blender and blend until smooth. Pour back into the baking dish.
Add the cooked pasta to the dish and stir well to coat. Mix in the spinach, freshly grated Parmesan cheese and a handful of chopped fresh herbs like basil or parsley. If the sauce feels too thick, stir in a splash of your reserved pasta water.
Spoon into bowls, top with extra Parmesan if you’d like, and serve warm!
