Heat oven to 350°F. Spray 13x9-inch (3-quart) glass baking dish with cooking spray. In 10-inch nonstick skillet, heat oil over medium-high heat. Add onion and garlic; cook 2 to 3 minutes, stirring occasionally, until tender.
Stir in frozen spinach; sprinkle with salt to taste. Cover; cook 5 to 6 minutes, stirring occasionally, until spinach is thawed and liquid has evaporated. Remove from heat. Stir in ricotta and Cheddar cheeses.
In large bowl, mix pasta sauce and tomatoes; spread about 1 1/2 cups in bottom of baking dish. With table knife, push about 1/4 cup spinach mixture into each uncooked manicotti pasta, pushing filling in from both ends. Place filled pasta diagonally over sauce in dish. Pour remaining sauce mixture over top.
Cover tightly with foil; bake 55 to 65 minutes or until pasta around outer edge is fork-tender. Uncover dish; sprinkle with mozzarella cheese. Bake uncovered 5 to 10 minutes longer or until cheese is melted and casserole is bubbly.