Preheat your oven to 400°F (200°C) and line a large baking sheet with parchment paper.
Clean the leeks thoroughly by slicing them into half-moons and soaking them in cold water to remove any grit, then pat them dry.
Place the chicken breast on one side of the baking sheet and the cleaned leeks on the other side.
Drizzle 0.5 tablespoons of olive oil over the chicken and leeks, then sprinkle with sea salt and black pepper.
Roast in the oven for 20 to 25 minutes, or until the chicken reaches an internal temperature of 165°F and the leeks are tender and slightly caramelized.
While the chicken roasts, prepare the sauce by heating the remaining 0.5 tablespoons of olive oil in a small skillet over medium heat.
Add the minced garlic and cook for 1 minute until fragrant, then whisk in the chicken broth, lemon juice, and fresh thyme.
Simmer the sauce for 3 to 4 minutes until slightly reduced, then stir in the fresh parsley.
Slice the roasted chicken and serve it over the leeks, drizzling the warm lemon-herb sauce generously over the top.