Preheat the oven to 400°F. Place the feta cheese in the center of an 8x11-inch baking dish. Add the onions and garlic around the feta. Drizzle the oil all over the feta and onions, then season with black pepper.

Bake for 30 minutes, until the onions are softened and the feta is melty and golden around the edges.

Meanwhile, bring a large pot of salted water to a boil. Cook the pasta according to package instructions until al dente. Reserve 1 ½ cups pasta water, then drain.

Remove the baking dish from the oven. Remove the garlic from the baking dish to cool slightly. Add the peas and basil directly on top of the hot feta mixture. Let sit for 2–3 minutes, until the peas soften from the heat.

Peel the garlic and return to the baking dish along with 1 cup of reserved pasta water. Use an immersion blender to blend everything together until smooth and creamy, adding more pasta water as needed to loosen the sauce. You can also transfer everything to a blender to blend if needed.

Add the cooked pasta directly to the baking dish and toss until the pasta is evenly coated in the sauce.

Serve warm with extra basil and a drizzle of olive oil, if desired.