Combine the cauliflower and cream in a large pot. Bring the cream to a boil, uncovered and cook until there is almost no liquid left and the cauliflower is just beginning to caramelize (this is key for a good texture – too much liquid results in a runny puree), about 30 minutes on medium-high heat.

While the cauliflower cook, caramelize the onion. Melt the butter in a medium pan. Add the chopped onion and cook, stirring occasionally until the onions are soft and golden, about 20 minutes. If they begin to catch or get dark in spots, add a bit of water and stir. When golden, remove from heat and set aside.

Add the cream cheese to the cauliflower and puree until smooth. Stir in half of the caramelized onions (reserve half for serving) and season to taste with salt and pepper, set aside. (Note that this can be made several days ahead and refrigerated.)

Combine the lupin flour, psyllium husk powder, and salt in a medium bowl and whisk to combine. Using an electric mixer, beat in the egg, cream, melted butter, and sour cream. Slowly pour in the boiling water while mixing. Continue to beat until the dough clumps together. Set aside for 5 minutes, it will be slightly sticky to the touch but should be like play dough.

Sprinkle the counter with a bit of lupin flour and roll 1-2 tablespoon size balls of dough into rounds about 1/16-⅛ inch thick. Cut into circles (either with a cutter or the rim of a glass).

Place a bit of filling in the middle of each round, fold in half over the filling, pressing the edges to seal (either with a dough press or using a fork).

Once all the pierogi are assembled, melt the butter for serving in a large nonstick pan. Add the pierogi and cook on medium-high heat until golden, about 5 minutes. Flip and cook the second side until golden, another 5 minutes or so. Add in the reserved caramelized onions and toss to coat. Serve with fresh herbs and sour cream, if desired.