Heat the oil in a large, heavy bottomed saucepan or Dutch oven over a medium high heat. Once it is shimmering, cook the onion with a generous pinch of salt for 8-10 minutes until soft, and just starting to color. Stir in the garlic, red pepper flakes and fennel seeds, and cook for a further minute until the garlic is soft and aromatic.
Crumble in the sausage, and cook, breaking it up with the back of your wooden spoon until it is cooked through and starting to color; this should take another 10 minutes.
Stir the tomatoes into the sausage, and season well with black pepper, and a little more salt. Do not discard the empty can.
Turn the heat up to high, then reduce to medium low, allowing the sauce to simmer for 20 minutes, periodically stirring and scraping any browned bits off the bottom of the pan into the sauce.
Pour the milk into the tomato can, and use it to wash any residual tomato juices into the sauce. Add the lemon zest and simmer for a final 20 minutes.
Meanwhile, cook the pasta as per the packets instructions, reserving a little of the pasta cooking water.
Add the drained pasta to the sauce, and stir until combined, adding a little pasta water if things look a little dry; you’re aiming for the sauce to coat the pasta with a glossy sauce.
Check to see if it needs any more salt and pepper before serving in warm bowls topped with the torn burrata and the shredded basil.