For a richer flavor, use homemade vegetable broth. You can also add a pinch of saffron threads to the broth for a more authentic paella taste. If you don't have merguez sausage, you can substitute with chorizo or other spicy sausage. Be careful not to overcook the seafood, as it will become tough and rubbery. The mussels are done when they open, and the shrimp are done when they turn pink and opaque. Garnish with fresh parsley or cilantro for a pop of color and freshness.