Heat the butter and oil together in a large skillet or saucepan over medium heat. Add the onion and sauté until translucent, 3 to 4 minutes. Add the rice and stir to coat in the oil and butter. Pour in the wine, then stir until all the liquid is absorbed. Reduce heat to low. Add 1 cup of the broth and cook until absorbed, stirring occasionally. Add the remaining broth, 1/2 cup at a time, waiting until each portion of broth is absorbed before adding the next. Continue this process until the rice is cooked al dente, about 40 minutes. Stir in the Parmesan, salt, and pepper. Puree 2 cups of the peas in a blender or food processor. Just before serving, stir in the pea puree until fully incorporated, thenfold in the remaining whole peas. Garnish with mint sprigs, if desired. Serve immediately.