Heat 1 tablespoon oil in a large cast-iron or nonstick skillet over medium heat. Add diced fennel and shallots; cook, stirring occasionally, until softened, 4 to 6 minutes. Add the remaining 1 tablespoon oil and the steamed vegetables; cook, stirring occasionally, until the vegetables are starting to brown, 4 to 6 minutes. Add garlic and cook, stirring, for 1 minute. Stir in salt and pepper and fennel fronds, if using.