Combine saffron threads and 3 tablespoons of warm (not boiling) water in a small bowl. Let steep for at least 10 minutes - the water should turn deep orange.

Rinse mussels under cold running water. Scrub off any barnacles with a stiff brush and pull off any beards by tugging sharply toward the hinge. Discard any mussels with cracked shells or any that don't close when tapped firmly.

Heat olive oil and butter in a wide Dutch oven over medium heat until the butter foams and subsides.

Add shallots and fennel with a pinch of salt. Cook, stirring occasionally, for 8 minutes until both are soft and the fennel edges are just starting to turn golden.

Add garlic and stir for 90 seconds until fragrant but not browned.

Pour in the white wine and let it bubble and reduce by half, about 3 minutes.

Add fish stock, crushed tomatoes, and the saffron-steeped water (threads and all). Stir to combine, bring to a gentle simmer, and cook uncovered for 5 minutes to let the flavors come together.

Increase heat to medium-high so the broth is actively simmering. Add all the cleaned mussels in a single layer as much as possible. Cover the pot immediately with a tight-fitting lid.

Cook for 4 to 6 minutes, lifting the lid at the 4-minute mark to check. Remove opened mussels with tongs to a warm bowl and replace the lid. Continue cooking until remaining mussels open, up to 7 minutes total. Discard any that haven't opened.

Remove the pot from heat. If using cream, stir it in now off the heat. Taste the broth and adjust salt - keep in mind the mussels have added brine.

Return all mussels to the pot or divide into deep bowls. Ladle broth and fennel over the top. Scatter parsley and reserved fennel fronds over each bowl. Serve with lemon wedges alongside.