Homemade spinach and sausage lasagna

INGREDIENTS
1 tablespoon olive oil
1 medium onion, diced
4 cloves garlic, minced
1 pound Italian sausage, crumbled
1 can (28 ounces) crushed tomatoes
1 teaspoon dried oregano (divided)
1 teaspoon dried basil (divided)
1 teaspoon dried thyme (divided)
1 (15-ounce) carton nonfat ricotta cheese
2 1/2 cups grated mozzarella cheese (reserve 1/2 cup for the top of the lasagna)
1/2 cup grated Parmesan cheese
1 package (10 ounces) frozen chopped spinach, thawed and squeezed dry
1 egg
9 lasagna noodles (cooked)

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