Gruyere and ricotta cheese create a creamy sauce for zucchini and summer squash.
Stir together Gruyère, ricotta, bread crumbs, cornstarch, basil, 3/4 teaspoon of the thyme and 1/2 teaspoon each of the salt and pepper; set aside.
Heat olive oil in a large skillet over medium-high heat. Add mushrooms and onion; sprinkle with remaining 3/4 teaspoon thyme and 1/4 teaspoon each salt and pepper. Cover; cook 6 minutes, stirring occasionally.
Coat slow cooker with cooking spray. Layer 1/3 of the squash in bottom. Add 1 cup mushroom mixture over squash; sprinkle 2/3 cup ricotta mixture on top. Layer with another 1/3 of squash, the rest of the mushrooms and another 2/3 cup of ricotta mixture. Place remaining squash and ricotta mixture on top.
Cover; cook for 2 hours on HIGH or 4 hours on LOW. Serve immediately.