Step 1

Preheat oven to 375 degrees. Butter a 2 1/2-quart casserole dish. Cook pasta in a large pot of salted water 4 minutes. Add cauliflower; cook until tender, 4 to 5 minutes more. Reserve 1/2 cup water; drain pasta mixture. In same pot, melt butter over medium heat. Add onion and celery, season with salt, and cook until onion is softened, about 8 minutes. Add flour; cook, stirring, 2 minutes. Slowly stir in milk until mixture is thickened, about 3 minutes. Remove from heat; stir in pasta mixture, tuna, and hot sauce. Season with salt and pepper. If mixture is too thick, add reserved pasta water.

Step 2

Transfer to prepared dish; cover with parchment-lined foil. Bake 15 minutes. Uncover, increase heat to 425 degrees, and bake until lightly browned, 10 to 12 minutes. Top with parsley; serve immediately.