Preheat oven to 350° F. Line a baking tray with aluminum foil or parchment paper. Set aside.
Cut broccoli florets lengthwise so they lay flat. Place cut florets into a large bowl ( or ziplock bag) and add olive oil, mix to thoroughly coat broccoli. Place broccoli on prepared baking sheet, making sure to leave room between easy floret so they can properly roast. Generously sprinkle with salt and course ground black pepper.
Place broccoli in oven and roast for 20 minutes( flipping half way through if you like). Sprinkle Parmesan cheese over broccoli and return to oven for 5 minutes.
While broccoli is cooking, prepare the lemon aioli by mixing all ingredients (mayonnaise, lemon juice, Dijon mustard, lemon zest, garlic powder and salt & pepper) together in a small bowl. Set aside.
Remove broccoli from oven and sprinkle with additional Parmesan cheese if desired.Serve with a dollop of lemon aioli on top or on the side( for dipping).