Preheat the oven to 350°F (175°C) and grease a 9x13-inch baking dish with cooking spray.
Bring a large pot of water to a boil and add 2 teaspoons salt.
Prepare the vegetables: trim snap peas, peel and slice the carrot, halve cherry tomatoes, and thinly slice mushrooms, mini bell peppers, and summer squash.
Cook the tortellini according to package directions; in the last 2 minutes of cooking, add the snap peas and sliced carrots. Drain and transfer the tortellini and vegetables to the prepared baking dish.
Heat olive oil in a skillet over medium-high heat and sauté mushrooms, mini bell peppers, and summer squash until crisp-tender, about 4–6 minutes. Pour the sautéed vegetables over the tortellini in the baking dish.
Add the halved cherry tomatoes and frozen corn to the casserole and gently toss to combine evenly.
In a small saucepan over medium heat, melt the butter. Whisk in the flour and cook 1 minute to form a roux.
Slowly whisk in the vegetable stock until smooth, then gradually whisk in the milk. Cook, stirring, until the sauce thickens.
Stir in roasted garlic powder, salt, pepper, dried basil, and Italian seasoning. Remove from heat and whisk in the softened cream cheese until the sauce is completely smooth.
Pour the cream sauce over the tortellini and vegetables in the baking dish and stir gently to combine and level the surface. Sprinkle the grated Parmesan evenly over the top.
Bake uncovered for 20–25 minutes, until heated through and the cheese is melted. If desired, broil 1–2 minutes to brown the top.
Let the casserole rest briefly, then serve hot with basil pesto on the side if using.
