Preheat the grill to 375 degrees Fahrenheit using indirect or two zone cooking. It’s important that the cast iron is not directly over the heat source or it will burn the broth. As you warm up your grill, be sure the cast iron, or grill safe pan, warms up with the grill.
When the grill reaches 375 degrees, place butter and olive oil in the pan. It should immediately start to melt. Add the shallots and stir frequently. This will cool the pan and allow the shallots to soften. Do this for 3 – 4 minutes or until the shallots start to get translucent. (note: this is also a good time to place the lemon half over direct heat to grill)
The garlic is one of the two main stars of this dish. Do not be shy with garlic! Garlic is your friend here. Continue to sauté the garlic for another 1 – 2 minutes.
Pour in the tomatoes and then wine, parsley, thyme, salt, and chili pepper flakes and stir for 2 minutes or until the wine starts to bubble. If the pan has cooled too much you can place over direct heat. But watch to be sure it doesn’t scorch over direct heat. (This is a good time to remove the lemon as it likely has a good char, then squeeze the juice into the pot – check out how to grill lemon if you have questions). This is when to add your bread over direct heat to get some char to it.
The grilled mussels will now go into the pan. Gently stir them with a large spoon to move them around and in the broth to coat all of the mussels with some of the sauce. Cover the mussels with a fitted lid, and then close the grill lid. Grill (over indirect heat) for about 6-8 minutes or until whole mussels open up. If the mussels were frozen and already in the half shell, then they should still cook in 6-8 minutes.
Remove the mussels (still covered) from the grill with heat appropriate gloves (the pot handle will be HOT). Serve them warm with crusty bread straight from the cast iron pan for an appetizer. If a main dish then place the mussels divided equally among four bowls and then pour in the broth.