Heat the oven to 350 °F, line a baking sheet with parchment paper, and spread the cauliflower florets out in one layer. The pieces should have a little space around them so they roast instead of steaming.

Drizzle the cauliflower with olive oil, toss to coat, and roast for 18 minutes. The edges should look slightly dry and lightly colored, not soft and wet.

While the cauliflower roasts, warm the heavy cream in a saucepan over medium heat and whisk often, then add the diced cream cheese. It is ready for the next step when you see small, lazy bubbles around the edge rather than a hard boil.

Turn the heat down to low and stir in 3/4 cup of the cheddar cheese along with the smoked gouda cheese. The sauce should loosen, then turn glossy as the cheese melts into the cream.

Add the garlic powder, ground mustard, sea salt, black pepper, and cayenne pepper if using, then cook for about 3 to 4 minutes until everything is melted. The finished sauce should look smooth and thick enough to coat a spoon without clumps.

Fold the roasted cauliflower into the sauce, transfer the mixture to a 9 x 9 casserole dish, and scatter the remaining 3/4 cup cheddar over the top. Make sure the florets are fully coated so the casserole bakes up creamy instead of patchy.

Bake for 20 minutes, then broil for a few minutes if you want more color on top. Stop broiling when the cheese turns deep golden brown at the edges, because it can go from perfect to burnt fast.