Chile rellenos are often fried; these peppers are filled with spiced rice and baked.
To make the filling, combine oil, onions, garlic, and rice in a medium saucepan over medium heat. Cook until onions are browned, 10 to 12 minutes. Mix in chili powder, and cook for 1 minute more.
Meanwhile, bring stock and salt to a simmer over medium heat. Add stock to rice, cover, and reduce heat to medium low. Cook for 25 to 30 minutes. Stir in acorn squash; add diced tomato without stirring. Cover, and cook 8 to 10 minutes more. Remove from heat, and let stand to steam for 5 minutes.
To make the chili sauce, heat oil in a medium saucepan over medium heat. Add onions and cook, stirring frequently, until browned, about 5 minutes. Add garlic, cumin, oregano, tomatoes, chili powder, salt, lime juice, and 1 cup water. Bring to a boil, and simmer for 5 to 8 minutes. Let cool slightly. Puree in a food processor or blender, and return to saucepan. If sauce is too thick, add a small amount of water.
Roast peppers over a gas flame or under a broiler until blackened, being careful not to break off stems. Place in a paper bag to cool. When cool enough to handle, peel peppers. Make a slit in the side of each from top to bottom. Wearing a rubber glove, scrape out the seeds. Stuff peppers with rice mixture.
Heat oven to 375 degrees. Place 1/4 cup chili sauce in the bottom of a 9-by-13-inch glass baking dish, and arrange stuffed peppers, cut sides up, on top. Cover with foil, and bake for 10 minutes. Garnish with cilantro, and serve with warm chili sauce.
Poblano peppers are readily available in the West and Southwest. Elsewhere, look for them in gourmet supermarkets and Latin groceries.