Vegetables à la Barigoule with Vanilla

Using a small sharp knife, cut all peel and white pith from lemon; discard. Cut flesh crosswise into 1/4"-thick slices; set aside.

Line a large rimmed baking sheet with a kitchen towel. Cook broccoli in a large pot of boiling salted water for 1 minute. Using a slotted spoon, transfer to prepared sheet. Repeat in same pot with carrots, onions, asparagus, and fennel, returning water to a boil between batches.

Place chicken broth in a small saucepan over low heat and scrape in seeds from vanilla bean; add bean. Bring to a simmer. Remove from heat; let infuse for 10 minutes.