Heat oven to 400 degrees.
Wash and peel celery root, potatoes, and apples.
Cut into large simarly sized chunks, 1" - 1.5", and set aside. Make the apple and celery root chunks the biggest, potatoes the smallest so the apple and celery root don't roast too quickly. (Place in a bowl of cold water to prevent oxidation.)
Mince garlic.
If using leeks, slice the white and light green parts into 1/2 inch pieces. (Optional)
Drain water from vegetables throughly and leave in the bowl.
Add olive oil, garlic, leeks, salt, pepper, sage powder, onion powder, and garlic powder to the celery root mixture and toss thoroughly.
Line two cookie sheets with parchment paper.
Place potatoes on cookie sheet and spread them out. They should not touch.
Place apple and celery root on the other cookie sheet and spread them out.
Add any leftover oil, garlic, or leek to either cookie sheet.
Place both cookie sheets in the preheated oven for 45 minutes, checking and tossing at the 20 minute mark.
Check the apple and celery root cookie sheet at the 30 minute mark for doneness. If roasted through to your liking, you can remove. If not, leave it in and continue to keep an eye on it. Remove at 45 minutes or when the potatoes are cooked through.
Place the roasted vegetables in a serving dish, sprinkle with additional salt and pepper to taste, and set aside.