Preheat the oven to 400F or 190C. Wash the apples, then dry them and cut them in half. Remove the core with a teaspoon or melon scooper.
Starting from the outer side, cut slices ¾ of the way through the apples, so that the apple is cut like a fan, but the slices are still attached together. My recommendation is not to make the slices too thin, or else the apple might fall apart while baking. The ideal thickness is ⅕ of an inch (or about ½ cm).
Combine together 2 tablespoons of sugar and 1 teaspoon of cinnamon, then add to the apples, in between the slices, and on top of the apples.Note: You don't have to add sugar in between all of the slices. Just the ones that are easier to handle.
Bake at 400F or 190C for 30 minutes. In the meantime prepare the streusel.
For the streusel
In a bowl, combine together flour, sugar, vegetable oil, rolled oats, cinnamon and salt. Mix with a spatula till you get a kind of a cookie dough.
Once the apples have baked for 30 minutes, take them out of the oven and apply the streusel, in between some of the apple slices, and some on top. Note: you can use a round-tip or butter knife to help yourself open the apple slices.Note: when you take apples out of the oven after the first 30 minutes have a look at the baking dish. If the bottom is dry, then add ½ cup of water, apple juice, or red wine. We noticed that some varieties of apples release less liquid while cooking and need some added.
Finish baking for another 20 minutes, then take out of the oven and serve. Optionally you can top hasselback apples with maple syrup, a scoop of ice cream or whipped cream.