Roast the chiles over a stove-top flame or a few inches beneath a broiler until charred, 4 to 5 minutes. When the chiles have cooled, peel off the skins and remove the seeds. Shred the chiles and set aside.
Blend the garlic and 1/2 cup water until smooth. Set aside.
In a large pot, cook the onion in oil over medium heat until tender, 8 to 10 minutes. Add the flour and cook 1 to 2 minutes more. The mixture will be thick.
In a blender, combine the tomatoes with their juice, the jalapenos and bouillon powder and blend until pureed. Add to the onion mixture. Add the garlic mixture, the remaining 4 cups water and salt to taste. Boil about 30 minutes. During the last 10 minutes, add the shredded chiles.
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