Preheat oven to 350°F.
In a large skillet over medium heat, combine the pancetta and onions and saute until the pancetta is crisp and the onions have softened.
Add the garlic and cook for 30-60 seconds until fragrant, then remove the skillet from the heat.
Lightly grease a medium casserole dish with olive oil, then toss in the cooked pasta, peas, pancetta-onion mixture and bechamel sauce.
Stir until the pasta is evenly coated in the sauce and the pancetta and peas are evenly distributed.
Top the casserole with Parmesan cheese and scatter the butter pieces over the top.
Bake for 15-20 minutes until the casserole is bubbling and golden on top.
Remove, let stand for 5 minutes, and then serve hot.
