In a small bowl, combine garlic powder with onion powder, 1/4 teaspoon Kosher salt, and 1/4 teaspoon ground black pepper. Lay the thighs atop a sheet of parchment paper to prevent contaminating kitchen surfaces and brush both sides with 1 tablespoon of olive oil between all of them. They should be coated lightly, NOT SATURATED. Sprinkle both sides of all of the chicken pieces with the dry mixture and allow to come to room temperature while you prepare a handful of radishes and red onions.
Use a sharp knife to cut the root tip from the radishes as well as the greens if they are present. Note that the greens may be incorporated into this recipe if desired, but may also be saved for salads or atop sandwiches as you would use lettuce.
Add the prepared radishes and red onions to a small bowl and toss with the remaining tablespoon of olive oil and some additional Kosher salt and black pepper. Arrange the chicken and vegetables in a single layer atop a large rimmed baking sheet, turning the fleshy side of the radishes cut side down against the metal. Turn the chicken skin side down against the metal.
Adjust a rack to the upper one third position and preheat your oven to 450°F. When the oven is fully preheated, place the baking sheet inside and roast for 20 minutes. At the 20-minute mark, use kitchen tongs to carefully loosen the chicken pieces from the sheet pan and flip them to skin side up. If using radish greens (optional), add them now.
Return the pan to the oven to continue roasting until the chicken reads 165°F when probed with an instant read thermometer. Be sure to take the temperature in the thickest part of the thigh without touching bone. At any point toward the end of the roast, you may use a spatula to loosen and turn the radishes to promote browning all over.
While the chicken and radishes roast, whisk together a half cup of crème fraîche with the Dijon, lemon zest, and lemon juice. Add freshly snipped chives and taste to adjust lemon zest or juice; add if required. Spoon into tiny bowls and scatter some capers and a light drizzle of olive oil atop each bowl of lemony creme fraiche. Serve with chicken and radishes.