Less a recipe, and more an algorithm. I've developed this method over many years of experimentation.
To start out, core and partially peel 4 or 5 nice-sized tart apples (I like Granny Smith or Macintosh). By partially peel, I mean take off about half the skin, in strips or spirals or stripes or however you want to do it, but leave some peel on for added flavor, texture, fiber and vitamins. (Of course, you can peel them all the way, if you'd prefer.) Slice the apples and scatter them in a 9" X 12" baking pan. Sprinkle the apples very thoroughly with cinnamon and maybe a little nutmeg (and I usually drizzle on some vanilla, too).
Then, in a large bowl, combine equal parts (I usually use 1 C of each, but you can go more or less depending on how much topping you want) whole wheat flour, rolled oats (sometimes I add one and a half parts of oats, for extra crunch), walnuts or pecans, and brown sugar, plus whatever else you've got laying around that might be good (slivered almonds, coconut, dried cherries, etc. etc.). Mix well, then cut in one part cold butter or margarine. Mush it all around until it's mostly the consistency of coarse crumbs, then spread this mixture evenly over the apples.
Bake at 350°F for 30 minutes or so, until the apples are soft. Serve warm, topped with milk, ice cream, whipped cream, soy milk, rice milk, or whatever. This makes a fabulous warming breakfast.
Note: The last time I made this, I realized halfway through making the topping that I had half as much butter as I actually wanted (because I was deciding at that moment that I wanted a double batch of topping because I had so many apples to cover). I made up the missing butter with applesauce (and I didn't add a double amount of sugar, to make up for the sugar in the sauce). It was delish!!
Another note: Lately I have been trying to reduce my sugar intake, and I've come up with a way to make this with "no sugar added" -- I slice up the apples into the pan, sprinkle them with spices and vanilla, then pour over them a full bottle of a complementarily-flavored diet soda (preferably from the local health food store, as most mainstream diet sodas have Nutrasweet in it, which I can't consume without raging headaches). I've been using Virgil's Diet Black Cherry Cream Soda with heartening results. Then I make the streusel with lots of flour, oats, nuts and butter but no sugar. Bake as usual. I'm impressed with how well this turns out!