Preheat oven to 400 degrees F. Line a baking sheet with parchment paper. Use a cutting board to slice the parsnips into thin ¼" circles. As you get towards the head of the parsnips, you may need to slice the circles in half to get a similar size. Toss with olive oil, salt, and pepper, and lay flat on the baking sheet. Bake for 10-15 minutes, or until slightly browned and tender.
In the meantime, prepare the guacamole. In a large bowl, combine the ripe avocados, onion, garlic, and lime juice. Smash the avocados with a fork or spoon. When the parsnips are finished cooking, add them in, and mix until well combined. Season with salt and pepper to taste. Enjoy!