Place half of the mango slices into the bowl of a blender with the coconut milk. Blend until smooth and reserve for later.
Heat the oil in a large pot over medium-high heat. Stir in the curry paste, and cook until fragrant, about 1 minute. Add the chicken and shallot; cook until the chicken is done and the shallots have softened, about 5 minutes. Pour in the mango puree, and cook until heated through. To serve, stir in the remaining mango slices and cucumber.