Mix chicken base with 2 cups water in small bowl. Set aside.
Heat 1 Tbsp olive oil in large Dutch oven over medium-high heat. When oil is shimmering, add sausage and cook until brown, breaking up pieces with wooden spoon, about 8–10 minutes. Remove from pan and set aside.
Add sliced shallots and garlic to the same pan from sausage. Sauté over medium heat until soft and translucent, being careful not to burn garlic, about 3 minutes. Add halved tomatoes and sauté until they begin to soften, about 3–5 minutes.
Add white wine to pan and simmer until almost evaporated, about 8 minutes. Return sausage to pan and add chicken base mixture. Increase heat to medium-high and bring to a simmer.
When sauce is simmering, add mussels and cover. Reduce heat to medium-low and continue to simmer until mussels open, 5–7 minutes. Be sure to allow mussels their full cooking time before checking on them. Peeking too early or often will let out the precious steam that helps them cook. Discard any mussels that do not open.
While mussels cook, halve ciabatta lengthwise. Toast on grill pan over medium heat or in oven until golden brown.
Split mussels and broth evenly between bowls, sprinkle fresh parsley over top, and serve with ciabatta alongside to dip in juice.
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