Cover eggs with cold water in a medium saucepan. Cook eggs, uncovered, at a gentle boil 10 minutes. Pour off hot water. Shake pan gently so eggs bump one another (to crack shells). Run cold water into pan to stop cooking. Let eggs stand in cold water 15 minutes, adding more water to keep cold.

Mince and mash garlic to a paste with a pinch of salt. Whisk together with mayonnaise, lemon juice, zest, mustard, 1/2 teaspoon salt, and 1/4 teaspoon pepper.

Chop eggs and stir into dressing with fennel and fronds.