Preheat the oven to 350F and line a baking sheet with parchment paper. Combine the butter and maple syrup in a small saucepan and set over medium heat until the butter is just melted. Remove from heat.
Combine the oats, nuts, salt, and zest in a medium mixing bowl. Pour in the butter mixture and stir to combine. Spread over the baking sheet and bake for 12 minutes, until golden, stirring the mixture once or twice as it bakes. Remove from the oven and let cool. (This can be prepared a few days ahead and kept in an airtight container. It is also great as a snack or breakfast treat, sprinkled over yoghurt.).
Clean the mixing bowl you used to prepare the topping and use it to combine the ricotta and sugar. Using a manual or electric whisk, beat the mixture until smooth. Add the cream and whisk again until thoroughly combined.
In a medium bowl, gently toss the diced mango with the lime juice. Divide the mango among glasses and top with the ricotta mixture. This can be prepared up to 4 hours ahead: cover and refrigerate until 10 minutes before serving.
Just before serving, sprinkle each glass with 2 tablespoons topping. Pour the rest of the topping in a small bowl and set it on the table so crunch enthusiasts can help themselves to more.