Preheat oven to 350°F and line a sheet pan with parchment paper.
Place the zest of 1 lemon and it's juice (about 3 Tbsp) in a small bowl. Add oil, garlic, salt and pepper. Mix well until salt dissolves. Add ½ of the fresh tarragon, saving the remainder for garnish.
Place trimmed asparagus in a bowl and spoon some of the marinade over top. Toss to combine and place on prepared sheet pan. Add leeks to the same bowl, toss with a little marinade and spread out on the sheet pan. Add the chicken breasts to the bowl, with remaining marinade until well coated. Nestle the chicken among the asparagus.
Zest the second lemon over the whole sheet pan and slice the lemons into rounds, layering them over the asparagus.
Bake 20 minutes, or until chicken reads 160°F.
Remove from oven and lightly toss, turning chicken over to coat the top with the juices. Sprinkle with remaining tarragon and serve.