Preheat the Oven: Preheat your oven to 375°F (190°C). Grease or line a muffin tin with paper liners.
- Combine Dry Ingredients: In a large bowl, whisk together the flour, baking powder, baking soda, salt, pepper, thyme, and garlic powder. Set aside.
- Mix Wet Ingredients: In a separate bowl, whisk together the milk, olive oil, egg, and Dijon mustard (if using).
- Add Veggies and Cheese: Stir the chopped spinach, red bell pepper, shredded cheddar, and Parmesan cheese into the wet ingredients.
- Combine Wet and Dry Ingredients: Add the wet ingredients to the dry ingredients and stir until just combined. Be careful not to overmix; it’s okay if the batter is a little lumpy.
- Fill Muffin Tin: Spoon the batter evenly into the prepared muffin tin, filling each cup about 3/4 full.
- Bake: Bake for 18-20 minutes, or until the tops are golden and a toothpick inserted into the center comes out clean.
- Cool and Serve: Let the muffins cool in the tin for 5 minutes before transferring to a wire rack to cool completely. Serve warm or at room temperature.