Easy Coconut Shrimp Curry

Name: Easy Coconut Shrimp Curry
Cuisine: Indian
Ingredients: Cooked rice for eating with curry; 1 lb raw shrimp OR chicken OR one package of firm tofu; 2 limes, juice of one lime about 1/4 cup lime juice plus one lime sliced for garnish*; salt and pepper; 2 tablespoon olive oil; 2 tablespoon butter ((optional but highly recommended)); 1/4 teaspoon cayenne pepper or dried Chile peppers (more or less to adjust to preference); 2 cups white, sweet or yellow onion (in chunks); 8 oz mushrooms (cleaned and sliced); 2 cloves garlic (minced); 1 tablespoon ginger (freshly grated or frozen minced); 1 14-15 oz can chopped tomatoes and juice (or about 2 cups chopped fresh tomatoes); 1 cup chopped green onions (green parts only (optional)); 1 teaspoon curry powder or curry paste ((if not using tumeric or coriander, use 1.5-2 tsp curry)); 1 teaspoon coriander; 1/2 teaspoon tumeric ((optional)); 1 can 13-14 oz can coconut milk (if using lite coconut milk, it will not be as thick); 1 tablespoon brown sugar ((optional)); cilantro (1/4 cup chopped for sauce, also extra chopped for garnish)