Remove kohlrabi stems and cut off tough outer skin, reserving leaves. Chiffonade kohlrabi leaves. Peel and core apple. Slice kohlrabi and apple into matchstick-size pieces. Chop enough fennel fronds to equal 1/4 cup and thinly slice the bulb lengthwise.

In a large bowl, whisk together oil, vinegar, mustard, shallot, honey, salt, and pepper. Add fennel and fronds, kohlrabi and leaves, apple, and pecans, and gently stir to coat. Season with salt and pepper to taste.

Serve chilled. Refrigerate for up to 1 day.