Some people I know can keep apples in their garage until mid winter. That never works for me and my cold cellar is too humid.
Can sliced apples be frozen to make tarts and desserts in the winter? If so, what is the process?
Wow, you're on a apple roll.
Sure can, and the process is easy. Your query reminded me of a pastry chef I worked with years ago, a young woman with tons of energy, who would prepare peeled, cored and sliced apples for pies, right down to the spices, sugar, lemon juice, and tossed with whatever thickener she used. She piled them into pie plates, wrapped them tightly with foil and froze them. When they were ready for baking, she'd take out the frozen apples en masse, pop them into pastry lined pie plates, added the top crust and bake, from frozen, not thawed. The baking time might be a bit longer, and you may have to tent your top crust with foil to prevent over browning. It worked amazingly well. She would even take the solidly frozen apples out of the pie plates, triple wrap them tightly in plastic wrap (this was before the advent of freezer zip bags) and store them in the freezer, stacked up.
Otherwise, I've frozen peeled, cored and sliced baking apples, tossed first with lemon juice (this prevents oxidation) and then tossed with sugar, or not, and packed into zip-loc freezer bags. Press out the air, seal, label and freeze. You can use these for tarts, apple crisp or cobbler, again from frozen, not thawed.
If your cold cellar is actually cold, I wouldn't worry about the humidity. Try wrapping each apple in newspaper, then placing it in a zipper plastic sandwich bag and seal almost completely. Put all the bags in an insulated cooler and place that in the coolest spot you have. I do this on my porch, which works until midwinter most years. Some varieties actually have made it all the way until April - these were not rotten but WERE too soft to eat. They were fine for applesauce. What you store and how successfully depends not only on climate control but on the variety you are keeping.