That can work. We’ve been doing something similar: It’s slow gains (~8kg/year) but it worked for us.

Changes were:

• Increase all meal sizes by ~50% - At least for us it meant every meal went from being “decently filling” to “very (but not overly!) filling”. Don’t repeatedly, find the lining where it still feels OK and don’t cross. There is a risk of loosing your body’s sense of appitite when crossing repeatedly which causes serious long-term problems (eating disorder).
• Use lots of unsaturated fats when cooking: Olive oil is great for this - It doesn’t add much subjective weight to cooked foods, has a lot of kJ/g and doesn’t cause any metabolic disorders if used in consumable quantities (unliked saturated acids or sugars).
• Bananas and peanut butter also rank high on the “decently healthy, high energy scale” and can be incorperated as/into daytime snaks (mini-meals).

All of this from some reading on the subject and self experimentation - I’m not a nutritionist, etc. 😊