Step 1
Place three 28-oz. cans complete peeled tomatoes with their juices in a big bowl, crushing together with your palms as you go; put aside.
Step 2
Prepare dinner 8 oz. smoked bacon (about 8 slices), finely chopped, in a medium Dutch oven or different heavy pot over medium warmth, stirring sometimes, till some fats has cooked out and bacon is browned however nonetheless mushy, 6–8 minutes. Utilizing a slotted spoon, switch bacon to a small bowl. Drain off all however 3 Tbsp. fats from pan; save for one more use or discard.
Step 3
Set pot over excessive and warmth fats till highly regarded. Organize 2 medium onions, peeled, halved by way of root ends, and 2 medium shallots, peeled, halved by way of root ends, minimize aspect down, in pan; prepare dinner till charred beneath, 2–3 minutes. Return bacon to pan and add 10–12 garlic cloves, coarsely chopped, one 7-oz. can chipotle chiles in adobo, chiles finely chopped, adobo sauce from can, 1 Tbsp. smoked paprika, 2 oz. basil (about half of a small bunch), and reserved tomatoes. Cut back warmth to medium and canopy, leaving lid barely askew. Prepare dinner, stirring sometimes to maintain sauce from sticking and adjusting warmth as wanted to keep up a mild simmer, till thickened, 2–2½ hours. Utilizing a slotted spoon or tongs, take away any huge items of onion and stringy basil stems; discard. Stir 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt into sauce; season with freshly floor pepper.
Step 4
When sauce is about 20 minutes away from being achieved, prepare dinner 1½ lb. spaghetti in a big pot of boiling salted water, stirring sometimes, till al dente. Drain, reserving 2 cups pasta cooking liquid.
Step 5
Add 5 Tbsp. unsalted butter, pasta, and 1 cup pasta cooking liquid to sauce and prepare dinner over medium warmth, stirring typically and including extra pasta cooking liquid as wanted, till butter is melted and included and noodles are coated in sauce and shiny.
Step 6
Divide pasta amongst plates or shallow bowls and prime with basil leaves.
