Heat some oil in a pan over medium-high heat. Add the diced onion and cook for 3-4 minutes until softened. Add the garlic and cook for 1-2 minutes until fragrant. Add the curry paste and cook for 1-2 minutes, stirring constantly. Sprinkle in the ground cumin, ground coriander, and turmeric, and cook for 1-2 minutes more. Pour in the coconut milk and bring to a gentle simmer. Simmer for 5-8 minutes until the sauce thickens and onions are completely cooked through. Season to taste with salt, pepper, and chili powder if desired.
Heat a grill pan over medium heat and coat with a thin layer of oil. Place the pork fillet slices in the pan and cook for 4-5 minutes on the first side, then flip and cook for 4-5 minutes on the second side, until the internal temperature reaches 63°C (145°F).
Toast the wholemeal bread slices. Place the toast on a plate and arrange the lettuce leaves on top. Arrange the cooked pork fillet slices on the lettuce, pour the curry sauce over them, and garnish with fresh coriander leaves.