Slice the runner beans very finely at an angle, then finely slice the fennel, reserving the fronds.
Rub the inside of a large bowl with the garlic. Add the oil, mustard, agave syrup, lemon juice and zest, and some seasoning. Stir well. Add the fennel and beans and toss everything together. Finely chop the fennel fronds and sprinkle over before serving.