Cut the bacon into a 1/4 inch dice and place in a cold saute pan. Start cooking the bacon over medium-low heat while stirring. The bacon will start to release fat into the pan as it cooks. Cook until the bacon is rendered and crispy, about 10 minutes.
When the bacon is rendered and there is some bacon fat in the pan, add the minced shallots. If very little bacon fat rendered out, add a tsp of olive oil to the pan.
Increase the heat to medium and allow the shallots to sweat in the bacon fat for about 30 seconds or until they are translucent but have not browned at all.
Add the spinach and allow to cook until it has wilted, about 1-2 minutes. Turn off the heat.
Let the spinach cool a bit before moving to the next step. If the color of the finished dish is important to you, you can set a metal bowl in an ice bath and place the cooked spinach mixture in here and stir it around until completely cool. The quick cooling will help preserve the bright green color. This step is not necessary for flavor.
Once the spinach mixture has cooled slightly (or completely if you are trying to preserve the color), give it a medium-fine chop on a cutting board
Place the chopped spinach mixture in a mixing bowl along with the grated garlic and parmesan cheese. Grate 1/4 of the lemon's zest and add it to the mixture.
Mix everything together and season with salt and pepper. The mixture should be pleasantly seasoned. I used about 1/3 tsp of kosher salt. This mixture can be used right away or kept in the refrigerator for up to 5 days. If refrigerating, temper the mixture so that it softens before spreading on the oysters.
Shuck the oysters and place them on an oven-safe tray lined with crumpled aluminum foil (make sure the tray fits in your grill if you are using a grill). The point of the crumpled foil is to keep the oysters level so that the butter and oyster juice do not spill out while cooking.
Spread about 1 Tbsp of the spinach mixture on each oyster in an even layer. I like to make sure the oysters are completely covered with the spinach. There should be enough spinach butter for 12 medium-large oysters. If the oysters are smaller, you may have enough for 18 oysters.
Preheat the oven to 350 degrees or light a charcoal grill with the coals set off to one side.
If you are using a grill, add a small handful of woodchips to the charcoal so that they begin to smoke and place the oysters offset from the fire if possible. You are looking for moderate heat. If using the oven, just put the oysters in the oven.
Close the lid to the grill or the oven door.
Allow the oysters to cook until the butter has melted and the oysters are just starting to bubble around the edges. This may take anywhere from 4 minutes to 15 minutes depending on the tempreature of the grill.
The butter should have melted and the oyster should feel medium-hot to the touch if you put a finger underneath the spinach mixture. You can also vary the amount of doneness on the oyster to your liking. If you want them to be very bubbly and the oysters to have firmed up a bit more, feel free to cook them longer. If you want the oysters just warmed through, cook them less.
Line a plate with seaweed, dry beans, crumpled foil, rock salt, or any other material to keep the oysters level. Top each oyster with a generous pinch of garlic breadrumbs and place the oysters on the plate with a few lemon wedges. Serve immediately.