Preheat oven to 425 degrees F. Spray a baking sheet with non-stick cooking spray and set aside.
Place chicken thighs in a large bowl and set aside.
Cut carrots into 1 1/2-inch pieces. Set aside.
Trim the stalks and fronds off the fennel, setting aside fronds. Cut the bulb into quarters through the root, then cut each quarter into 1 -inch slices. Mince ~1T fennel fronds. Add fennel slices and minced fronds to bowl with chicken.
Mince 3 cloves of garlic and add to bowl with chicken. Add 6 fingerling potatoes to bowl.
Add juice of 1 lemon + 2T olive oil to bowl and season w/ S&P and toss to combine.
Transfer the chicken mixture onto a large baking sheet. Arrange the vegetables around the outside and place the chicken pieces closely together in the center. Pour any remaining juices from the bowl over the chicken.
Roast until the chicken reaches an internal temperature of about 165°F and the vegetables are tender and beginning to brown around the edges, about 30 minutes. Toss vegetables halfway through cooking.