Heat 2 teaspoons oil in a medium heavy frying pan over medium heat. Add tomatoes and cook for 3 mins or until tender. Add 3/4 cup of the stock and cook for 1 min or until the tomatoes are rehydrated. Transfer mixture to a blender. Add the creme fraiche and lemon zest, and puree until smooth. Cover to keep warm.
Meanwhile, heat a large heavy frying pan over medium-high heat. Pound the chicken to 1cm-thick, then season. Add the remaining oil to the hot pan, then add the chicken breasts and cook for 5 mins each side or until golden brown and just cooked. Transfer the chicken to a plate to rest and keep warm.
Add the leek to the frying pan and cook for 3 mins or until caramelised. Add the asparagus, peas and sugar snap peas and cook for 3 mins or until just tender. Season and add lemon juice to taste. Divide among plates.
Slice the chicken and arrange over the vegetables. Spoon the tomato sauce over each chicken breast and around the plate.
Make the sauce up to 1 day ahead, then cover and refrigerate. Reheat the sauce, covered, in a small heavy saucepan over low heat just before serving.