Preheat the oven to 400°F / 200°C.
Steam the cauliflower florets for 5 to 8 minutes until crisp-tender.
Drain thoroughly and place in a large casserole dish or ovenproof skillet.
While the cauliflower steams, prepare the marinara sauce.
Slice most of the pepperoni, reserving 10-15 whole slices for the topping.
In a large bowl, combine the marinara sauce and room-temperature cream cheese.
Pour the mixture over the cooked cauliflower.
Add the sliced jalapeño, sliced pepperoni, oregano, red pepper flakes, and most of the mozzarella and Parmesan cheeses to the dish, reserving some of each for the topping.
Mix until all the cauliflower is well-coated.
Season with salt and pepper to taste.
Top the casserole with the reserved cheeses and the whole pepperoni slices.
Bake for 15 to 18 minutes, or until the top is bubbly and lightly browned.
Remove from the oven and let it cool for a few minutes.
Garnish with fresh basil before serving.