Turn the chicken breast-side down. Use kitchen shears to cut out the backbone of the chicken. Flip the chicken back over and press it down flat with the palms of your hands against the cutting board to open it up like a book. Place the chicken in a large zip-top bag.
Whisk together the olive oil, fennel seeds, thyme, garlic, lemon zest and lemon juice and season with salt and pepper. Pour the marinade over the chicken and place in the refrigerator for at least 3 hours and up to 1 day.
Preheat the grill to medium-high heat.
Use a clean tea towel and wipe the grill grates with oil. Remove the chicken from the marinade and place skin-side down on the grill and close the lid. Grill until the skin is nice and golden and crisp, about 15 minutes.
Once the chicken has nice golden grill marks, turn the grill to medium-low heat. Flip the chicken again, close the grill lid and continue cooking until a thermometer reaches 165 degrees F when inserted into the thickest part of the thigh, 20 to 25 minutes longer. Check the chicken occasionally to make sure it's not burning and rotate as necessary.
Cut into 6 to 8 pieces before serving. Serve the chicken on a platter with the Simple Green Salad.
Simple Green Salad:
Add the greens to large bowl. Add the mustard and lemon juice to another bowl. Drizzle in the olive oil while whisking to emulsify. Season the dressing with salt and pepper. Toss the greens with the dressing and transfer to a platter.