Preheat oven to 425. Line a baking sheet with parchment paper. Wash your radishes and cut off stems/leaves. Slice each one in half, and in half again if they're very large. Place into a bowl. Cut onion into 1/2 inch-1 inch pieces and add to the bowl, along with the olive oil, garlic, salt, and thyme. Use hands to mix well.
Spread the radishes and onions out onto the baking pan evenly. Roast for 25-30 minutes until fork tender, flipping halfway through. Enjoy with your favorite protein!