Begin by gathering your vegetables for the Casserole. Chop the bell peppers, onions, zucchini, and mushrooms into bite-sized pieces. Rinse the spinach and set it aside. Mince the garlic until fine.

In a large skillet, heat olive oil over medium heat. Add onions and minced garlic, sauté for 2-3 minutes until fragrant. Then add bell peppers, zucchini, and mushrooms, cooking for another 5-7 minutes.

In a slow cooker, layer the sautéed vegetables as the base. Add fresh spinach, followed by diced tomatoes, vegetable broth, thyme, and oregano, stirring to combine.

Cover the slow cooker with the lid and set on low heat for 4-6 hours, until the vegetables are tender.

About 10 minutes before serving, stir in the grated cheddar cheese until melted and bubbly.