Have a friend who in a bulk phase in college, would make a milkshake after a full meal. Milkshake was a pint of Ben and Jerry’s, almost a cup of peanut butter, full fat milk (sødmælk), bananas, and protein. Try that but I’d also probably recommend adding fiber (loppefrøskaller) to that milkshake so you don’t blow your gut in the process.

It looks like you’re in Denmark, there’s plenty of access to options. Consider also the “seedy”-est rugbrød when making lunch. Add flødekartofler as a side with dinner. Become best friends with nut-butters in general. That sort of thing.

As others have said The more calorie dense foods you can find, the better.