Preheat your oven to 375°F (190°C) and grease a 9x13 inch baking dish.
In a large bowl, whisk together flour, baking powder, baking soda, salt, thyme, and rosemary. Add the cold, cubed butter to the flour mixture. Using a pastry cutter or your fingers, cut the butter into the flour until the mixture resembles coarse crumbs. Gradually stir in the buttermilk until just combined. Do not overmix. Set aside.
In a large skillet, heat olive oil over medium heat. Add the diced onion and minced garlic, sautéing until the onion is translucent (about 3-4 minutes). Add the bell pepper, zucchini, carrot, and spinach to the skillet. Stir in oregano, salt, and pepper. Cook for an additional 5-7 minutes or until the vegetables are just tender.
Pour the sautéed veggies into the prepared baking dish. If using, sprinkle half of the shredded cheddar cheese over the vegetables. Pour the vegetable broth evenly over the veggie layer to ensure they steam properly while baking.
Take your biscuit mixture and dollop spoonfuls evenly over the top of the veggies. It’s okay if some of the vegetables peek through; this gives a rustic look. If you prefer, sprinkle the remaining cheddar cheese over the biscuit topping.
Place the casserole in the preheated oven and bake for 25-30 minutes, or until the biscuit topping is golden brown and a toothpick inserted into the biscuit comes out clean.
Allow the casserole to cool for a few minutes before serving. Cut into squares and enjoy this delightful holiday dish!
Notes
For a dairy-free version, substitute butter and buttermilk with plant-based alternatives.