Melt butter in a frypan over medium heat. Swirl olive oil into melted butter. Cook garlic, eschallots and capers in oil and butter until eschallots are translucent, about 5 minutes. Mix in tomato sauce, Italian herbs and chilli flakes; stir until well incorporated. Cover and reduce heat to medium-low; allow to simmer for 10 minutes.
Add mussels to the pan and cover tightly. Raise heat to medium-high and cook until all mussels have opened, about 10 minutes. Remove from heat and garnish with chopped spring onions to serve. Discard any unopened mussels.