Half or quarter 2 cups radishes according to size, making them even for uniform cooking.
In a sheet pan, toss the cut radishes and 1/3 cup onions with 2 tablespoons duck fat (or olive oil), 1/4 teaspoon fine salt and 1 teaspoon freshly ground white pepper.
Roast in the oven for about 35-40 minutes at 220 °C (425 °F). Midway through cooking, open the oven and stir the radishes .
Remove from the oven when the radishes are shriveled and soft. Test with a fork, they should be tender all the way through.
Spread 1 cup fresh mozzarella on top of the radishes and take it back into the oven until it's fully melted.
Notes
You can add a different, easy melt cheese (other than mozzarella) if you prefer.They are best served immediately. If not, please see post for storage and reheating tips.Garnish suggestions: sour cream or ranch seasoning, chopped parsley or green onions