Heat vegetable broth in a saucepan over medium heat. Sauté onions and tomato for 2 to 3 minutes.
Add garlic for 1 minute. Reduce heat and simmer for 5 minutes, stirring occasionally.
Add some more vegetable broth if necessary, so mixture doesn’t become dry. Add masala seasoning, curry, saffron, and sea salt (if desired). Simmer for 10 minutes more. Remove from heat and cool for several minutes.
Pour mixture into a blender bowl and puree on medium until smooth and creamy.
Return mixture to saucepan and add coconut milk. Bring to a mild boil, then reduce heat to a simmer for 5 minutes. Remove from heat.
While marinade is cooling, lightly pound chicken breasts with a meat mallet.
When marinade cools, place in a bowl and add chicken to sauce. Cover and refrigerate for at least 2 hours, up to 24 hours.