Heat a cast-iron skillet over medium-high heat until hot. Add the jalapeno slices to the dry skillet and cook until just starting to blacken. Set aside to cool.
Mix together the sour cream, cumin, cayenne pepper, green onions, cooled jalapenos and 2 1/2 cups of the grated Cheddar in a bowl.
Using tongs, char the tortillas one at a time over a gas flame for 5 to 10 seconds per side, then dunk each tortilla in the Enchilada Sauce. Lay a tortilla on a plate, then spoon a good 2 to 3 tablespoons of the sour cream mixture in the middle. Roll the tortilla, then place it seam-side down in a 9-by-13-inch baking pan. Repeat with the remaining tortillas and sour cream mixture.
Cover the dish with the remaining 1 1/2 cups cheese, then bake until bubbly, 15 to 20 minutes. Sprinkle with extra green onions and serve immediately.
Heat the oil in a medium saucepan over medium heat. Add the peppers, onions and garlic and cook until soft, about 2 minutes. Add the chili powder, cumin, cayenne, bouillon cubes and some salt and pepper and cook for 2 minutes. Add the flour and cook for another minute. Add the stock and stir to combine. Add the tomato sauce and 3 cups water and bring to a boil. Lower the heat to a simmer and cook until reduced by one-third, about 20 minutes.
Let the mixture cool slightly, then transfer to a blender and puree until smooth.
The sauce will keep in the refrigerator for up to a week and can also be frozen.
You can heat the tortillas in a microwave on a paper towel if you don't have a gas stove.
When blending hot liquid, first let it cool for five minutes or so, then transfer it to a blender, filling only halfway. Put the lid on, leaving one corner open. Cover the lid with a kitchen towel to catch splatters, and pulse until smooth.