Place the bacon in a 5-6-quart pot over medium heat and cook until the fat has rendered, about 5 minutes. Add the onions, salt, and pepper and sweat, stirring frequently, 5 minutes. Add the butter and melt, then pile on the potatoes and fennel, without stirring. Pour in the broth and the vermouth (or wine) then arrange the cod filets (still frozen is fine) on top along with the lemon wheels.
Cover with a lid and simmer over medium-low heat for 20 minutes or until fish is cooked through. Remove and discard the lemons and break up the fish a bit, stirring into the stew. Add the half and half and drop the heat to low and barely simmer (do not boil) for another 20 minutes, uncovered.
Correct the seasoning with more salt and pepper and, if desired, stir in some mashed potatoes, a spoonful at a time, to smooth the texture (especially helpful if the dairy has slightly curdled).
Serve immediately or better yet, refrigerate and serve the next day, giving the flavors time to meld. Garnish, if you wish, with finely chopped fennel fronds. The chowder also freezes well in airtight containment for up to 3 months.