Peel and finely chop the onions and the garlic cloves. Add them to a deep pan and leave to warm up for a minute while stirring continuously. Add 50 ml of water, season with a bit of salt and leave to simmer for 10 minutes.
In the meantime, boil the pasta according to the package instructions. Tip: use an electric kettle to boil the water faster, then pour the boiling water to the pot you want to boil the pasta in.
Add the tomato passata, the pepper paste, the lemon juice and zest and leave to simmer for 15 minutes. Stir from time to time.
Add the smoked salmon cut in smaller chunks (no need to finely chop it, it will fall apart anyway when cooked). Add the capers and leave to simmer for 5 more minutes.