Preheat oven to 375 F
Bring a large pot of water to a boil. Add 1 tsp Sea Salt, Cauliflower Florets, and Potatoes and cook on a medium heat for 7-8 minutes. Drain.
In a medium sized saucepan and over medium heat, melt 2 tbsp butter, add flour and whisk to combine. Add warm milk, stirring constantly until the sauce thickens.
Remove pan from heat. Add the remaining 1/4 tsp of sea salt, pepper, 1/4 cup Gruyere cheese, Parmesan cheese, and nutmeg.
Taste and add more salt if needed.
Transfer cauliflower and potatoes into the 8x11 inch baking dish. Cover with sauce and carefully mix to combine the ingredients.
Sprinkle breadcrumbs and the remaining 1/4 cup of Gruyere on top of the gratin and drizzle with 2 tbsp of melted butter.
Bake for 25-30 minutes until vegetables are tender.
Turn on the broiler and bake for an additional 2-3 minutes or until the top of the gratin is golden brown.
Serve immediately. Enjoy!