Brown the ground beef in a large, heavy saucepan over medium heat. Add the onions and cook, stirring, until lightly golden and soft, about 10 minutes. Sprinkle the mixture with the chili powder and flour and stir to blend. Add the tomatoes, garlic and enough water to cover the chili. Stir well. Bring to a boil, then reduce heat to medium-low and simmer, covered, stirring occasionally.
Toast the chipotles in a hot skillet. When they swell with air, use a spatula to press them down. You will see a wisp of smoke leave the skillet. Place the toasted chipotles in a bowl and cover with water. After about 30 minutes, remove the chilis and reserve the water. Remove the stems and seeds, dice the chipotles, and add to the chili. Add the tomato paste to the chipotle water and mix well. Stir the tomato paste mixture into the chili. Simmer chili covered, for about 30 minutes, adding more water if necessary the mixture becomes too thick.
Makes 8 to 10 servings