Heat oven to 450 degrees. Pulse bread, 2 tbsp cheddar, 2 tbsp mont jack, 2 tbsp butter, 1 clove minced garlic, 1/4 tsp salt,and 1/4 tsp pepper in food processor until coarsely ground.
Heat remaining butter, remaining garlic, and flour in dutch oven over med heat, stirring constantly until golden and fragrant, about 1 min. Slowly whisk in cream and broth. Stir in cauliflower, dry mustard, thyme, salt and pepper and bring to a boil. Reduce heat to med-low and simmer covered, stirring occasionally , until cauliflower is nearly tender, 6-8 min. Off heat, stir in remaining cheddar and Mont jack until incorporated. Pour mixture into 9x13 baking dish and top with bread crumbs. Bake until crumbs are golden brown and crisp, 10-15 min. Let cool 10 min