Cut fennel bulb in half through the root. Shave or thinly slice bulb (perpendicular to cut made to half the bulb). Do the same with the shallots or onion if using. Set aside.
Cut bacon strips in half lenghtwise, then cut into 1/4 inch strips. Add to heated pan and cook until fat is rendered. Remove from pan and set aside.
Leave bacon fat in pan (you can leave it all in the pan or remove as much as you want, but leave at least enough to coat the bottom of the pan).
Add fennel and shallot/onion to heated pan. Cook until just past translucent (or carmelized if you want).
Add spinach to pan with fennel and shallot/onion. Cook until spinach has given up all of its liquid.
Add bacon back to pan with spinach mixture. Add salt and pepper to taste.
Serve immediately or keep warm in oven (although you shouldn't make this too long before you're ready to serve).
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