Build a hot charcoal fire in your Weber Charcoal Summit Grill, on the highest burn grate, so the coals are close to the cooking grate. Place a Lodge 12” cast iron skillet in the cooker to preheat. Add the bacon to the hot skillet. Cook until the bacon begins to brown, about five minutes then add the white parts of the green onions, the shallots and a couple of tablespoons of the cajun seasoning. Cook, stirring occasionally, until the bacon is crisp and the shallots have softened, about 10 minutes total.
Add the garlic, cook for one minute, then add the mussels and beer and cook, uncovered, until the mussels begin to open, about 2 minutes. Add the butter and tomatoes. Continue cooking, stirring, until the butter has melted and the mussels have opened completely, a few minutes more. Taste sauce and season with salt as needed. Add the green parts of the sliced green onions. Serve with the sliced baguette.