Question about weight gain "cheat codes"

Ho! Im 26M, European, and my main symptom si nausea.

My weight has been stable for about a year, but I'm underweight (I've lost 15–20 kg the first year where my symptoms started) and now im not taking any medications.

What are your "cheat codes" that have helped you maintain or gain weight? Easy calories, incredibly calorie-dense and tolerable foods, or anything else that comes to mind.

Thanks for the responses and sorry for any translation error!