1/4 cup vegetable oil, divided
1/2 cup hazelnuts (2 ounces), coarsely chopped
4 bacon slices
1 tablespoon Sherry vinegar or red-wine vinegar
1 tablespoon fresh lemon juice
4 medium radishes, thinly sliced
Heat 1 tablespoon oil in a small skillet over medium heat until it shimmers.
Add hazelnuts and cook, stirring occasionally, until golden, 2 to 3 minutes.
Cool slightly in skillet.
Cook bacon in a large skillet over medium heat until crisp.
Transfer to paper towels to drain and reserve 1 tablespoon bacon fat. Break bacon into 1-inch pieces.
Purée hazelnuts (with any remaining cooking oil), vinegar, lemon juice, 1/4 teaspoon salt, remaining vegetable oil, and reserved bacon fat in a blender just until nuts are finely chopped (dressing will be thick and slightly chunky).
Toss frisée, radishes, and bacon with dressing.