In a bowl, mix the sour cream and the mustard, salt and pepper. Cut the tomatoes in slices and flake the canned tuna with a fork.
Unfold the puff pastry and spread it out with a roll of pastry to make it thin. Spread the mustard/sour cream preparation on the puff pastry. Place tomatoes slices, tuna, olives and oregano evenly on the puff pastry. Fold 1 inch of the edges of the tart and brush them with the egg yolk.
Place in the oven 30 minutes. Check after 20 minutes. Edges should be golden brown.
Drizzle with olive oil before serving. Serve with a green salad