Heat oil in a large skillet over medium-high heat. Add radishes and cook, stirring often, until radishes begin to brown, 2 to 4 minutes. Add thyme, rosemary, and garlic. Cover and reduce heat to medium-low. Cook, stirring occasionally, until radishes are tender when pierced with a fork, 6 to 8 minutes. Add butter and salt; cook, stirring constantly, until butter has melted, about 1 minute.