Cook pasta in boiling salted water just until tender (al dente). Reserve 1/2 cup cooking water, then drain pasta.
Heat oil in a skillet. Add onions and garlic. Cook until fragrant and translucent. Add choice of add-ins like sliced cherry tomatoes and black olives. I also added some scallions for color. Cook until slightly softened.
Add canned sardines along with its oil. Sprinkle salt and red pepper flakes. Cook for 1 to 2 minutes. minute.
Add the cooked pasta and a splash of pasta water, about 1/4 cup to 1/2 cup. Toss well with a tong to combine. Turn off the heat. Taste and adjust salt and lemon juice as needed. Sprinkle grated parmesan cheese and parsley. Serve immediately.
Notes
Canned Sardines in oil: use your favorite brand. Remove the bones by splitting the meat into two and picking the bones with a fork; use boneless canned sardines if available.
Add-ins: I added cherry tomatoes and black olives. You can also add capers, spinach or kale for some greens.