Heat oven to 450°. Meanwhile, toss chicken with 2 t salt and 1 t sugar in a medium bowl. Set aside.
In a large bowl, toss veggies (squash, brussels sprouts & onion) with olive oil, garlic, 2 t of the salt, and pepper. Then toss the chicken in with everything.
Transfer everything to a lipped baking sheet or shallow pan. Position chicken in the middle with the veggies scattered around. Sprinkle the lemon juice and the leaves from the rosemary sprigs atop everything. Cook for 20 minutes. [Chicken is considered fully cooked at 165°].
Video
Notes
Heat oven as first step to stay on track with time.
Buy a small squash. You won't want to cut a big ol' squash.
To prep the squash, cut the squash length-wise, run a spoon along the middle of each section to scoop out all the seeds and gunk, and slice squash (skin facing up) into ¼" cuts.
If the chicken is a little fatty, use kitchen shears to clip off the excess fat.
Chicken is done when its temperature is at least 165°F.