Soak skewers in hot water or soak overnight.
Blanch the Fingerling Potato (8 ounce) in a pot of salted simmering water, until fork tender, for about 15 minutes (depending on the size). Drain.
Cut Leek (2) into disks about 3/4-inch thick.
Set leeks in a saute pan with 1/2 inch of water (coming up half way up the sides of the leeks) bring to a boil, turn heat down and simmer on low for 5 minutes, until tender, and vibrant green. Drain, carefully, so they stay intact.
Cut Boneless, Skinless Chicken Breast (1 pound) into 1-inch cubes. Generously salt and pepper the chicken on all sides.
In a small bowl, mix Dijon Mustard (4 tablespoon), Olive Oil (4 tablespoon), Garlic (3 clove), Fresh Rosemary (1 tablespoon), and Salt and Pepper (1/2 teaspoon) until incorporated.
Preheat the grill to medium high. Slice the Lemon (1) and begin assembling the skewers: chicken, potato, leek through the side, not middle, a lemon slice folded in half, and repeat, chicken, potato, leek and finish with chicken on the end.
Brush each skewer with the dijon marinade on all sides, using up all the marinade.
On medium high grill , grease with spray oil, and place skewers on the grill. Cover and let cook about 4 minutes, or until there are good grill marks.
Turn over. Turn heat to medium, cover and cook 4 more minutes. Rotate and grill the other two sides of the chicken, for a few minutes.
Turn heat off and let chicken rest on the grill, covered, allowing it to continue to cook through until its done, about 5 minutes.
Place on a platter and scatter lemon slices and rosemary sprigs around. Serve with a Ceasar Salad.