Thinly slice the fennel and onion. Keep the fronds.
Heat the olive oil in a frying pan. Add the fennel and onion and sauté, under cover, for 5 minutes. Add the dried herbs and lemon juice, season to taste, then cover again and cook for another 5 minutes until soft. (Add a spoonful of the pasta’s cooking water if it’s getting too dry).
Meanwhile, cook the pasta according to the packet instructions, then drain, reserving about a cup of the cooking water.
Mix the pesto with the lemon zest, some cooking water and lemon juice.
When the fennel & onion are soft, gently mix in the pasta, pesto and a spoonful of the cooking water.
Toss well, then serve with the tomatoes & plenty of freshly ground pepper. Garnish with basil leaves.