150 ml (1/4 pint) extra-virgin olive oil, plus extra if needed
Brush the tomatoes with oil and season well. Place on a nonstick baking sheet and cook in a preheated oven, 220°C (425°F), Gas Mark 7, for 10–12 minutes.
Meanwhile, lay a chicken breast between 2 sheets of clingfilm and flatten slightly with a rolling pin or meat mallet. Repeat with the remaining chicken breasts. Brush lightly with oil and season. Heat a griddle pan until smoking hot, add the chicken and cook for 5–6 minutes on each side or until cooked through. Remove the chicken and leave to rest for 2–3 minutes.
While the chicken is cooking, make the pesto. Put all the ingredients in a food processor or blender and blend until fairly smooth, adding a little more oil for a runnier consistency if liked.
Transfer the chicken on to warm serving plates, drizzle over the pesto and serve with the roasted vine tomatoes.