Preheat oven to 450º Fahrenheit. Wash 2 bunches radishes under running water, scrubbing radishes with vegetable brush to remove any surface dirt. Once washed, pat radishes completely dry with paper towels and place radishes on cutting board.
Slice off and discard stems of radishes and tail on root (if any). Cut radishes into equally-sized pieces, halving and quartering radishes as needed.
Spread radishes across baking sheet. Drizzle 1 tablespoon avocado oil over radish pieces and sprinkle 1 tablespoon salt and 1 tablespoon freshly cracked black pepper over pieces. Toss radishes to fully coat in oil and spices.
Place baking sheet in preheated oven. Roast radishes 20 to 30 minutes, stirring radishes every 10 minutes, until radishes are browned in spots and slightly wrinkled. Be careful not to burn radishes.
While radishes roast, place small saucepan or skillet on stovetop over medium heat. Add 2 tablespoons unsalted butter to pan. Simmer butter, watching closely, 3 to 5 minutes or until butter is deep, toasty brown color with nutty fragrance. Remove from heat immediately when color deepens and fragrance develops.
Set pan aside and let butter rest 2 to 3 minutes. After 2 to 3 minutes, add 1 teaspoon minced garlic and 1 pinch salt. Stir until garlic is softened and fragrant. Set aside.
When radishes are ready, remove pan from oven. Immediately pour browned butter evenly over roasted radishes, then toss radishes in butter until thoroughly coated.
Transfer roasted radishes to serving bowl. Garnish with ¼ teaspoon fresh lemon zest and 1 sprig fresh thyme if desired. Serve immediately.