In a large, tall saucepan, cook bacon lardons over medium-high heat until rendered, 12-15 minutes. Using a slotted spoon, remove cooked bacon and set aside, leaving bacon fat in the pan.
Add onion and cook, stirring occasionally, until lightly caramelized, about 5 minutes. Stir in garlic, serrano chili and bay leaves and cook until fragrant, about 1 minute. Using your hands, crush whole peeled tomatoes over the pan. Add the liquid from the tomato can as well, stirring to incorporate.
Add pinto and navy beans, dijon mustard, smoked paprika, dry mustard and bacon to the pan, stirring well to incorporate. Pour in the vanilla porter, adding a small amount of water if the mixture is not fully covered, season with salt and pepper and stir well. Increase heat to bring beans to a boil, then reduce to a simmer. Cover and simmer, stirring occasionally, for 90 minutes.
After 90 minutes has elapsed, remove lid and increase heat to reduce the bean mixture, stirring frequently to avoid burning. Remove from heat and allow to cool after mixture has thickened considerably into a chili-like consistency, 15 to 20 minutes. Serve garnished with sliced scallions.