Preheat oven to 375°. Set beet in a small baking pan with 1/4 in. water and cover tightly with foil. Roast until tender when pierced, 1 to 1 1/4 hours.
Let beet cool; peel and quarter. Purée in a food processor until smooth, scraping inside of bowl as needed.
To beet, add chickpeas and reserved liquid, tahini, garlic, zest and juice of 1/2 lemon, 1/2 tsp. salt, and 1/4 tsp. pepper; whirl until smooth. Drizzle in 1/4 cup oil, pulsing to combine.
Add more lemon juice, salt, pepper, or oil to taste. If hummus is too thick, add a bit of water. Scrape into a bowl and top with chervil sprigs.
Make ahead: Up to 2 days, chilled.