Preheat the oven to 400°F.
Add the whole tomatoes, garlic and chipotle to a blender and puree until smooth. Add to a saucepan and simmer over medium heat until slightly thickened, about 15 minutes.
Heat the olive oil in a large skillet over medium-high heat. Add the onions and bell peppers and saute until softened, about 5 minutes. Add the plum tomatoes and cook until it breaks down, another 3 minutes. Stir in the beans, quinoa and chili powder. Turn off the heat and fold in the chopped green onions if using.
Split the poblano peppers in half and remove the seeds. Stuff the peppers with the quinoa mixture. Ladle about half of the tomato sauce into a 13 by 9-inch casserole dish. Place the peppers on top and ladle over the remaining sauce. Sprinkle the peppers with the cheese, cover the casserole dish with foil and bake for 30 minutes. Remove the foil from the top and cook until the peppers are very soft, another 5 minutes. Top with Avocado Cream Sauce, diced tomatoes and sliced green onions if desired.
Avocado Cream Sauce
Slice the avocado in half and remove the pit. Scoop the flesh into a food processor and add the yogurt, lime juice and a big pinch of salt and pepper. Blend until smooth.