Preheat the oven to 450 degrees Fahrenheit. Line a baking sheet with parchment paper. (Use 2 if necessary.)
Place the whole poblano peppers and onions slices on the baking sheet. Then place the cut vegetables--tomatoes, serrano pepper and garlic bulb--on the baking sheet, cut side down. Roast for 25-30 minutes, turning the peppers and onions over halfway through cooking, until the peppers are blistered on both sides the veggies are cooked.
Optional: Make the Cheddar Sauce if using. Set aside.
To make the filling: While the vegetables are roasting, in a pot or large skillet, cook the spinach, and drain the excess liquid. Add the cooked quinoa, black beans, corn, and taco seasoning, and cook a few minutes over medium heat, just enough to heat and blend the flavors and spices. Add a little water if needed to keep things from sticking--but allow it to cook off so the filling is not too wet. Season with sea salt and freshly ground black pepper, to taste. Set aside.
When the veggies finish roasting, remove them from the oven, and reduce the heat to 400 degrees Fahrenheit.
To make the sauce: Use a spatula to add the tomatoes, onions and serrano pepper to a blender. Use a towel to grasp the garlic bulb, squeeze the cloves out of their skins, and add them to the blender. Add the tomato paste and cilantro to the blender, and process on low, gradually increasing to high speed, being careful to let some steam escape. Add sea salt to taste. Pour the sauce into the bottom of a baking dish just large enough for the peppers, about 13 x 9 inches.
Place the poblano peppers on a cutting board. Carefully slice one side of each pepper down the center lengthwise to form a pocket, being sure not to pierce the other side. Use a spoon (or your fingers) to gently scoop the seeds and membrane out of each pepper.
Fill each pepper with 1/4 of the filling, and place into the baking dish, cut side up.
Cover and bake 20-25 minutes at 400 degrees Fahrenheit, or until the filling is hot and the sauce is bubbling.
If desired, drizzle gently warmed cheddar sauce over the top. (See notes)
To make your own taco seasoning, combine: 1/2 teaspoon chili powder, 1/4 teaspoon ground cumin, 1/8 teaspoon oregano, 1/8 teaspoon onion powder, 1 pinch of cayenne, garlic powder and sea salt.
I always keep cooked grains around, and I like quinoa in this recipe for its texture and protein. But if you want to skip the step of cooking quinoa, you can simply increase the other filling ingredients. You'll need about 3 to 3 1/2 cup to fill the peppers.
Don't overheat the cheddar sauce. Cook in a sauce pan on medium low heat, stirring frequently so that it doesn’t separate.