Thaw the flounder, if using frozen, and preheat oven to 400 degrees. Combine the melted butter, juice from half of the lemon, minced garlic, pepper, parsley, and smoked paprika in a small bowl.
Cut one large piece of foil for each fillet, leaving enough room to fold over both fish and asparagus. Divide asparagus and fish fillets evenly, then place one fillet and a few spears of asparagus side-by-side in the center of each piece of foil.
Spoon the seasoned garlic butter mixture evenly over the flounder & asparagus.
Fold the foil over each packet and seal it tightly across the top, so that none of the butter leaks out.
Place packets on a rimmed baking sheet and bake at 400 degrees for 15 minutes, or until fish is cooked through and flakes evenly with a fork, then open foil and sprinkle Parmesan evenly across the fish and asparagus.
Return to oven and broil on high with the foil open for 2 minutes to melt the cheese on nicely, then squeeze on additional lemon juice to taste (it really brightens up the flavor!).
Serve the fish and asparagus with its own sauce over either rice, or riced cauliflower.
Note: If you can't find flounder, another firm white fish will do -- I've also made this with pollock and it came out great. If you don't choose to bake in foil you should also be able to use parchment paper in this recipe; just be sure to seal as tightly as possible with the seam on top to avoid the butter leaking out. I added the Parmesan at the end rather than cooking it in with the fish and asparagus, just so that it wouldn't stick to the inside of the packet.
Easy Flounder & Asparagus Packets from Mashup Mom: https://www.mashupmom.com/easy-flounder-asparagus-packets/