How to Make It
Preheat oven to 450°F with rack in upper third of oven. Cut each cauliflower head vertically through core into 2 to 3 (3/4-inch-thick) steaks to form 8 steaks. (Reserve remaining cauliflower for another use.) Arrange steaks in a single layer on a large rimmed baking sheet. Brush oil evenly on both sides of steaks.
Cook fennel seeds in a small saucepan over medium-high, stirring occasionally, until fragrant, about 1 minute. Transfer to a mortar, and crush using a pestle (or coarsely grind in a spice grinder). Stir in salt, pepper, orange zest, and garlic powder. Sprinkle mixture evenly over both sides of cauliflower steaks.
Roast cauliflower steaks in preheated oven until golden brown and tender, about 30 minutes, flipping halfway through roasting time. Remove from oven; increase oven temperature to broil.
Spread cauliflower steaks evenly with marinara; top with cheeses. Broil until cheeses melt and start to brown, 6 to 8 minutes.