Melt butter with oil in a large skillet over medium-high heat. Add onion and cook, stirring often, until softened, 3 to 5 minutes. Add peas and stir until warmed through, about 3 minutes.
Before serving, add mint and ricotta salata to pea mixture. Toss to blend; season with salt and pepper. Serve warm or at room temperature.
Find recipes with ingredients that you have on hand.