Because it dries out fast, keep the phyllo dough packaged or fully covered until Step 8. Keep the camembert in the fridge until Step 7. (It’s easier to slice when it’s chilled.)
Wash and dry the asparagus. Cut each stalk into thirds, discarding the woody ends, reserving the top two portions to be used in the tart.
Preheat the oven to 375F.
Spray a 9x13 glass casserole dish with nonstick cooking spray. Set aside.
In a large mixing bowl, whisk together the sour cream, egg, thyme, salt, and pepper until smooth. Set aside.
Shred or grate the fresh parmesan. Set aside.
Unwrap the camembert and slice the whole wheel into ¼-inch slices. Set aside.
Melt the butter in a microwave-safe dish. Set aside.
Remove the phyllo dough sheets from the package. Working very quickly, fold them like you might a napkin and begin stacking on the skinny side of the casserole dish. You’re going for a loose, messy, accordion-style, “file folder” appearance. They may begin to slide because of the nonstick spray, but we will straighten them up as we tuck cheese and asparagus into the folds. (See photos above for help.)
Once all the phyllo is arranged in the casserole dish, begin tucking in portions of the asparagus deeply throughout until all of the asparagus is used.
Do the same for slices of the camembert, tucking a slice here and there, between folds, like a bookmark in a book. If the phyllo sheets have slid so that they are laying flat, lift and rearrange. The presence of cheese, asparagus and other sheets of phyllo should help hold these layers up as you build.
Once the dish is filled with phyllo, camembert and asparagus: Use a pastry brush to "paint" the melted butter all over the surface.
Pour the egg and sour cream mixture evenly over the surface, using a bowl scraper to get out as much as can and smooth it over the surface of the folds.
Dust the top with the shredded Parmesan. I measure cheese with my heart, so go with what your heart tells you here.
Bake for about 15 to 20 minutes. A knife in the center should come out clean.
Cool for two minutes, then slice into squares and serve warm. (Though it’s pretty good at room temp, too, honestly.)
For small tastings (3x3-in squares), this will serve about 12. For hearty portions, plan for 6 servings.