In a large nonstick skillet, cook pancetta over medium, stirring occasionally, until crisp, 8 to 10 minutes. With a slotted spoon, transfer pancetta to paper towels to drain. Pour off all but 1/4 cup drippings (if necessary, add enough oil to measure 1/4 cup). Add brussels sprouts and cook, stirring occasionally, until crisp-tender, 10 minutes. Stir in rosemary and cook until fragrant, about 2 minutes. Stir in pancetta. Remove from heat, stir in lemon juice, and season with salt and pepper. Serve warm or at room temperature.