Preheat oven to 400°F. In a large oven-safe skillet add 1 tbsp olive oil over medium-high heat.
Add fennel, season with salt and freshly ground black pepper and sautée until golden-brown and caramelised on all sides, about 10 minutes. Add lemon juice and pour the chicken or vegetable stock in the skillet and cover with a lid. Cook the fennel for 3-4 minutes or until tender and can be easily pierced with a fork. Remove fennel and juices from skillet and set aside.
Wipe the skillet with a paper towel. Heat remaining 1 tbsp olive oil, season chicken with salt and freshly ground black pepper and cook until golden-brown, about 5-7 minutes on each side. Transfer skillet to the oven and bake for 15-20 minutes or until the internal temperature reaches 165°F.
When the chicken is done, add tomatoes, garlic, fennel and cook for 1-2 minutes.
Serve with fennel fronds, flaky sea salt and freshly ground black pepper.