Preheat the oven to 350 F degrees and lightly grease your skillet.
Peel and cork the apples then slice them into semi-thick chunks, approximately the size of the blueberry. Zest the lemon peel then juice the lemon.
In a medium to large bowl, add the filling ingredients, which include apple slices, blueberries, lemon juice and zest, sugar maple syrup, corn starch, all spice, cinnamon and nutmeg. Mix gently until all the ingredients are mixed and everything is coated with maple syrup.
Pour the filling mixture into the skillet and spread it so it is an even layer.
Wipe the bowl clean then add the topping ingredients, which include rolled oats, flour, melted butter, brown sugar, walnuts and salt into it. Mix until everything is fully combined. The crisp will look slightly wet but not completely drenched.
Sprinkle the topping on top of the filling until it is properly covered. 90% or more of the filling should be covered.
Place into the oven and bake for 30-35 minutes. The crisp should be golden brown and the filling should be bubbling.
Let cool for five minutes, then serve! Option to top with whip cream and ice cream.
This crisp will last up to 3 days in an air tight container in the fridge