Prepare your vegetables. Trim the root ends and tips from the vegetables. Rinse and scrub the carrots and celery. Dry well. If desired, peel your carrots before chopping.
Chop your vegetables. The size you choose to chop your onions, carrots, and celery will vary depending on the recipe being prepared. However, as a general guideline: for sauteed recipes, chop small (¼-inch to ½-inch), for soups and stews, medium size (½-inch to 1-inch), and for stock, broth, or blended soups, larger size (1-2 inches).
Cook vegetables over medium-low heat. Add the fat to a large skillet set over medium-low heat. Add the fennel bulb, carrots, celery, along with the horseradish and lemon grass; mixing well to coat. Cook, stirring often, for 10-12 minutes, or until softened and onions are translucent. Adjust heat to prevent vegetables from browning