Put the dressing ingredients into a small food processor and process until smooth and creamy. Taste to adjust any of the ingredients. Note: jalapeños vary in their level of heat, but generally one pepper will give you a mild to moderate heat, and two will up the spice factor. I used 1 1/2 peppers, with the seeds and veins. I thinly sliced the remaining half pepper for garnish.
Quarter the cabbage and remove the core. Then thinly shred the quarters. Peel and quarter the onion, then thinly slice it (you may not need the whole onion.) Trim the fennel bulb and slice in half, remove the core, and thinly slice. I like to reserve some of the fennel fronds for a garnish.
Put all the veggies in a large bowl and toss with enough dressing to coat everything well.
Refrigerate until ready to serve. Toss again before serving, and add more dressing if you like. Garnish with fennel fronds and thinly sliced jalapeños if you like.