In a large bowl, mix together the radishes, onions, peppers, garlic, salt and pepper. Ensure all the vegetables are about the same size so that they will cook evenly. Pour over the melted butter and mix well.
Pour the mixture directly into the Air Fryer basket.
Set the Air Fryer to 360F for 20 minutes. Then, increase the temperature to 400F for 3 minutes to crisp up the edges of the vegetables.
If you want to make this in the oven, use the broil setting on your oven. You will need to watch the radishes carefully to ensure they don't burn, stirring from time to time.
You can also try making this in a nonstick pan. In that case, add 2-3 tablespoons of oil or ghee to the pan, and then basically pan-fry your veggies in the oil on high heat until they crisp up.
Unless you like the occasional and awkward crunch of an uncooked vegetable, try your best to make sure that all the veggies are cut to about the same size. This will help them all cook evenly.
Radishes make a wonderful keto substitute for potatoes, so don't be afraid to try them in other traditional potato recipes!