Heat the oven. Preheat to 200°C (400°F).
Assemble the bake. Set the feta in the middle of a deep baking dish. Scatter the cherry tomatoes and garlic around it. Drizzle with olive oil and sprinkle over the Italian seasoning, chili flakes, salt, and pepper.
(Optional step to replace a large piece of feta: I cube my feta up before baking - find it easier to mix around and I end up with little lumps of feta - YUM)
Roast, 25–30 minutes. Bake until the tomatoes have burst and collapsed and the feta is soft enough to smoosh with a fork.
Cook the pasta. While the feta bakes, cook the penne in salted boiling water until al dente. Scoop out 1 cup of pasta water before draining.
Mash the sauce. Pull the dish from the oven and mash the feta and tomatoes together with a fork, right in the dish, until you have a rough, creamy sauce.
Combine. Add the drained pasta and spinach to the dish and toss well, loosening with splashes of reserved pasta water until everything is glossy and coated.
Finish and serve. Stir through the basil and lemon zest if using. Taste, adjust the salt and pepper, and serve warm.