Preheat oven to 425°F (220°C). Line a baking sheet with parchment paper.
Remove sardines from the can and reserve the oil. Gently flatten the sardines slightly and place them in a single layer on the baking sheet.
Bake sardines for about 15 minutes until crisp around the edges.
Bring a large pot of salted water to a boil and cook thin spaghetti until al dente. Reserve ⅓ cup pasta water before draining.
Heat the reserved sardine oil and olive oil in a large pan over medium-low heat.
Add sliced garlic and capers. Cook gently for 1–2 minutes until fragrant and lightly golden.
Add cooked spaghetti to the pan and toss well.
Pour in pasta water, lemon juice, Parmesan cheese, and parsley. Toss until glossy and combined.
Gently mix in the crispy sardines or place them on top.
Finish with extra Parmesan, parsley, and lemon juice if desired.