Allow cauliflower florets to thaw, then chop them into very small pieces.
Preheat the oven to 425 F.
Line a baking sheet with parchment paper.
Placed chilled leftover mashed potato in a large bowl along with chopped cauliflower, shredded cheese, garlic powder, smoked paprika, and salt. Stir until everything is evenly combined. Taste mixture and adjust salt or spices if needed.
Briefly whisk the egg in a small bowl, then add it to mashed potato mixture. Stir until it is fully incorporated into mashed potatoes. The mixture will be wet but not very sticky.
Place mixture in a baking pan. Baked it for 15-20 mins or more until you achieve the golden brown color on top.