@Patrick66 , how about hot chocolate, made with cocoa powder, double cream and water, plus sweetener if you prefer it. Loads of calories, packed into a cup or mug.
Just try to keep fairly straight until this op is done. You're unlikely to become medically, concerningly underweight in the intervening period.
Please don't go overboard on the breaded/battered stuff short term, because the A1c is weighted; reflecting the most recent blood glucose performance with more a reference to the older stuff. This is why some folks can see significant improvements to their A1c score in a brief period.
Once your op is out of the way,one big, longer terms stressor will be removed from your life, leaving more energy and head space to deal with the rest of life. Don't under estimate the impact of stress, and this op has been grumbling on for a long time.
Please view eating as a job of work. For the vast majority's of the time, I really enjoy my food, but when eating to a goal (in this instance focusing on gaining weight), it can become a chore, but some chores have to be done.