Bring a large pan of water to the boil. Add the potatoes.Simmer for 15-20 mins till just tender. Drain. When cool enough, slice them thinly.
While the potatoes cook, peel and finely slice the onion. Slice the garlic. Drizzle 1 tbsp olive oil into a small ovenproof frying pan. Add the onion and garlic. Cook gently for about 10 mins till softened.
Beat the eggs in a bowl. Cut the salami in strips. Grate the cheese. Roughly chop the parsley leaves and stalks. Add all of this to the beaten eggs with some pepper (you probably won’t need any salt because of the cheese and salami).
Tip the onion into the egg mixture and add the sliced potato. Carefully mix together to distribute everything evenly. Tip back into the frying pan and give it a gentle shake to level out (or see our tip below).
Heat the grill. Cook on the hob on a fairly low heat for 7-8 mins till the bottom of the frittata is set. Sit under the grill and cook for another 7-8 mins till the top is golden brown.
Halve the tomatoes. Season with some salt and pepper. Finely snip the chives. Scatter over the tomatoes. Dress with ½ tbsp olive oil. Serve with slices of frittata.