Roast the chiles over an open flame, or under the broiler until the skin is blackened and blistered, turning to roast on all sides.
Place the chiles in a plastic bag for 5 minutes. Peel off the blackened skin. Remove the seeds and cut the chiles in thin strips.
Place the sour cream, milk, flour, garlic and bouillon cube in a food processor. Cover and process until smooth.
Heat the oil in a 10-inch skillet over medium-high heat. Sauté the onion until soft and translucent. Add the chiles and stir until heated through. Reduce the heat to low and stir in the sour cream mixture. Cook for 5 minutes or until thick and bubbly, stirring frequently.
Remove from the heat and sprinkle the chiles with the cheese. Cover and let stand for 1 minute or until the cheese is melted.