To brown the butter: Place three pounds of butter in a sauce pot over medium high heat. Skim off any froth that comes to the surface throughout the cooking process. It should take about 15 - 30 minutes, depending on heat, before the butter starts to brown. Once the butter changes from bright yellow to a golden brown colour and begins to smell nutty, turn off the heat. You don't want it to burn. Skim off any more foam on the top, and discard. Strain brown butter through a fine mesh strainer. Place in the fridge until it has hardened.
Combine remaining ingredients in a large bowl.
Once brown butter has hardened, measure out the required 1.25 lbs and cut into 1 inch cubes. Toss the butter into the flour mixture, pressing it between your thumb and fingertips until all the butter is pea-sized and no large lumps remain. This should take about 1 - 3 minutes. Do not over-work or your topping will be too dense.
Store the topping in the fridge or freezer while you make the apple filling.
Peel, core and slice the apples to about 1/4" thick.
Toss all ingredients together, and spread out evenly in a greased 9x13 baking dish.
Spread crumble topping evenly over the top of the apples.
(If you have any left over brown butter, you can place small cubes of it over the top between the apples before putting the crumble topping on, and it will melt down into the apples as they cook for even more brown butter flavor!)
Bake at 400F for 10 minutes. Drop the temperature to 325F and continue baking for 45 minutes to an hour depending on how "al dente" you prefer your apples.