Apple Crisp

What I would normally do when I feel like baking but can't come up with any idea of what to make is simply to choose something plain and easy...This time I thought about rhubarb to begin with but since I didn't have any of them in hand, so my aim was changed to apple which I happened to have enough for the dish!

I slightly adapted the recipe used for my rhubarb crisp . This time I reduced amount of topping and used lemon instead of orange..

Here is what I've done

114 g cold butter, cut into small pieces, plus more for greasing pan
6 cups  apple ( half granny smith half gala ) peeled,cored and cut into 1 inch pieces (about 6 apples)
2 tbsp caster sugar
1 tablespoon lemon juice
a zest from one (medium) lemon
1/4 scant cup brown sugar
3 tbsp caster sugar 
1/2 cup + 3 scant tbsp all-purpose flour
1/2 rounded tsp cinnamon
1/4 tsp salt
3/4 cup rolled oats

1. Heat oven to 375 degrees. Grease an 8- or 9-inch square baking or gratin dish with a little butter. Toss apple with white sugar, lemon juice and zest, and spread in baking dish.

2. Put the sugars, flour, oat, cinnamon and salt in a mixing bowl, rub in the butter until it looks like small peas and just begins to clump together. Crumble the topping over the apple and bake until golden and beginning to brown, 30 to 40 minutes. Served warm with cream or ice-cream.