In a 6- to 8-quart pan over high heat, bring 3 quarts water to a boil. Meanwhile, trim and discard leaves from cauliflower; separate heads into florets. Slice florets 1/4 inch thick. Add sliced cauliflower to boiling water and cook until tender-crisp, 2 to 3 minutes. Drain and rinse under cold water; drain well.
In a glass measure, stir together cream and cayenne. Line bottom of a shallow 3 1/2- to 4-quart baking dish with a fourth of the cauliflower slices. Drizzle a fourth of the cream mixture over cauliflower. Sprinkle lightly with salt and pepper and top evenly with a fourth of the poblano chile strips and cotija cheese. Repeat three times to layer gratin, ending with cheese.
Bake in a 450° oven until cheese is browned and cauliflower is tender when pierced, about 30 minutes. Let stand 10 to 15 minutes before serving.