Preparation Arrange racks in upper and lower thirds of oven; preheat to 450°F. Toss panko, Parmesan, cheddar, and 2 Tbsp. butter in a medium bowl until combined. Sprinkle evenly onto a parchment-lined rimmed baking sheet. Bake on lower rack until cheese is melted and bubbling and panko is deep golden brown, 6–8 minutes. Let cool. Crumble into small pieces; set aside until ready to use. Meanwhile, cut cauliflower into 1/2"-thick steaks (it’s okay if the outside ends break apart). Whisk garlic, thyme, and remaining 1 cup butter in a medium measuring cup. Toss half of cauliflower, half of shallots, 1/2 cup butter mixture, 4 tsp. salt, and 1 tsp. pepper on a rimmed baking sheet until coated. Repeat with remaining cauliflower, shallots, 1/2 cup butter mixture, 4 tsp. salt, and 1 tsp. pepper on another rimmed baking sheet. Roast, tossing veggies and rotating sheets from top to bottom halfway through, until tender and browned, 45–55 minutes. Let cool slightly. Transfer to a platter. Top with reserved crumble and parsley. Serve with lemon wedges for squeezing over. Do Ahead: Uncrumbled panko mixture can be baked 1 day ahead. Wrap baking sheet with plastic and store at room temperature.
Preparation Arrange racks in upper and lower thirds of oven; preheat to 450°F. Toss panko, Parmesan, cheddar, and 2 Tbsp. butter in a medium bowl until combined. Sprinkle evenly onto a parchment-lined rimmed baking sheet. Bake on lower rack until cheese is melted and bubbling and panko is deep golden brown, 6–8 minutes. Let cool. Crumble into small pieces; set aside until ready to use. Meanwhile, cut cauliflower into 1/2"-thick steaks (it’s okay if the outside ends break apart). Whisk garlic, thyme, and remaining 1 cup butter in a medium measuring cup. Toss half of cauliflower, half of shallots, 1/2 cup butter mixture, 4 tsp. salt, and 1 tsp. pepper on a rimmed baking sheet until coated. Repeat with remaining cauliflower, shallots, 1/2 cup butter mixture, 4 tsp. salt, and 1 tsp. pepper on another rimmed baking sheet. Roast, tossing veggies and rotating sheets from top to bottom halfway through, until tender and browned, 45–55 minutes. Let cool slightly. Transfer to a platter. Top with reserved crumble and parsley. Serve with lemon wedges for squeezing over. Do Ahead: Uncrumbled panko mixture can be baked 1 day ahead. Wrap baking sheet with plastic and store at room temperature.