any tips for weight gain without satuated fats

I really should be seeing a Dietician but I guess I'm lazy so I'll see what the HC lifestylers have to say -

I was 64kgs bang on for 12 years, I ate alot of food even tried to put on weight many times as 64kgs / 5'10" is right on borderline however super fast metabolism, even if I ate a ridiculous amount of food at dinner & built a big belly by the time I wake up it's back to bang on 64kgs - & I did eat alot of fat in my diet in attempt to raise the my weight.

Anyway down to the crunch about a year ago I had a burst stomach ulcer (perforated duodenal ulcer) that was incredibly excruciating pain when it did break & emergency surgery to flush & patch it - i was then in hospital for 8 days following it fed off a drip.

When i left hospital 56kgs but the same conundrum over the past 12 months I haven't been able to put on any weight to get back to 64kgs - regardless of what i eat it's still all gone when i wake up & bang on 56kgs.

For other health reasons I'm removing saturated fats from my diet (it's been a week now) so just looking for some tips to gain weight but it must be without satuated fats.

That's my weekend question for ya lol :-)