In a large pot, melt the butter. Add the bacon and cook over moderate heat, stirring occasionally, until browned and crispy, about 5 minutes. Add the onion and garlic and cook over moderate heat, stirring occasionally, until translucent. Sprinkle the flour over the top and then whisk until a paste forms. Cook, stirring occasionally, until it smells toasty, about 1 minute. Gradually stream in the milk while whisking constantly. This is important to make sure there are no lumps in your gravy. Cook over moderately low heat, whisking frequently, until the gravy begins to thicken, about 10 minutes. The gravy should just coat the back of a spoon when it is done. Stir in the jalapeño and lemon juice and season with salt and pepper. Keep warm over low heat.