Add chopped fennel and olive oil to blender. Using plunger attachment, pulse, then puree on medium low. Increase speed to high, remove plunger, and cover with lid. Blend on high until fennel is very smooth and cooked through, five minutes.
With blender off, carefully remove lid and scrape down sides and lid. Mixture will be very hot.
Add cream cheese and feta. Blend on high until smooth, 30 seconds. Turn off.
Add spinach and use plunger to tamp leaves down into mixture. Pulse very quickly two to three times, just until spinach is chopped through. Place in serving dish and garnish with toasted whole fennel seeds and fennel fronds if using. Can be served warm or chilled.