Process tomatoes in a blender or food processor until smooth, stopping to scrape down sides.
Melt 1/4 cup butter in a Dutch oven over medium-high heat; add onions, and cook, stirring often, 30 to 40 minutes or until golden brown. Stir in pureed tomatoes, beef consommé, and next 3 ingredients; bring to a boil. Reduce heat, and simmer, stirring occasionally, 10 minutes.
Ladle soup into 8 ovenproof bowls; top with bread slices, and sprinkle evenly with cheeses.
Broil, 5 1/2 inches from heat, 3 minutes or until cheese is browned and bubbly.