Gaining

Double cream or whipping cream

I have no tips sorry, but if that’s your genetics it may be hard to fight. My father and other sibling are skinny like 10 to 12 stone. So I just have fat gene’s. All you can do is eat as much as you can and do not exercise much.

Heavy cream, as others have said is pretty much the pinnacle. It has one of the highest calorie counts for the money. Yet is still palatable and digestible.
Other things that can help is mixing carbs and fats. Pasta with butter. Lots of butter. Just be careful with carbs and sugars as they lead more toward visceral fat that is the kind that causes health issues. Going back and forth between starving yourself and eating like crazy can confuse your metabolism a bit.
I was skinny and even underweight before trying to gain most my life. Gaining weight is hard work! Lol. It's 3500 calories for just 1 lb of body fat. Then that fat is distributed all over your body. So it takes a lot to be noticeable. Keep track of how many calories you eat. You need about 2000-2500 a day just to exist. Then going above that on a consistent basis. It will also take time. Gaining is a marathon not a sprint. Just like losing. Count calories to see how much extra you eat a day and you can then do the math to see how much you will gain and how long it will take. It's usually almost 10lbs to be noticeable, depending on you body. There are calorie and gaining calculators online. You don't so much have to ruin your metabolism but to beat it into submission .
I think many people don't realize how hard gaining can be and how long it takes. I know I didn't lol. But don't let that discourage you. Just temper your expectations. Keep at it and before you know it a month will have passed and you've gained 15-20lbs. Even if it's less, a few months can pass pretty fast and those calories add up. Best of luck! Hope that helps a little.