In a small bowl, combine the parsley, chives, and tarragon. Whisk in sour cream and both mustards. Stir until blended.
In a large dutch oven, melt the butter under high heat. Add garlic and thyme and stir for about 1 minute. Add mussels, tomatoes, and green onions. Slowly pour in the beer and add in sour cream mixture. Season with salt and pepper and mix well. Cover and steam for about 8-10 minutes, or until mussels open. (Discard any mussels that have not opened after 10 minutes, they're no good)
Serve mussels in a deep bowl and top with croutons. (As you can see, I went a little crouton crazy, but I like my croutons!) Dig on in, and just to warn you, it's going to be messy, but you'll enjoy the flavor too much to care!