Hi hi! I've found that it was easy for me to add a decent amount of calories per meal by incorporating fats in carb-heavy dishes, as Munchies suggested.
Here's what I did: I would cook things with lentils as a main ingredient pretty often, and in an appropriate step of making the dish I would add quite a bit of coconut oil. Coconut oil is apparently easier to digest, the thinking was then that extra hundred calories per tbs wouldn't be so hard on the body, making it a more meaningful calorie boost. Some lentil dishes I did this with was vegetarian chili, veg tacos, or plain old beans and rice which is just brown rice and lentils boiled together with some spices, add cheese, butter, it's easy.
I started my list with lentils because they are so, so cheap, they're versatile, and many people find them delicious. Some other tried and true dishes to bolster calories is the classic pasta alfredo, the sauce is butter cream flour and cheese, it is the easiest thing to make and downright delicious. I love to make noodles by hand, the alfredo sauce from scratch, and bake homemade rolls. Of course adding butter and jam onto the homemade rolls, it is all quite decadent although labor intensive to make at home! I don't cook it for myself at least. Jars of alfredo sauce keep for a long time, so does boxed pasta, and it's cheap. If you make mac n cheese, do what Munchies said, add cream instead of milk!! Or half&half. It is so true that cream goes well into pasta dishes!
My suggestions for adding calories by this method: sauces/pasta dishes, soups/curry/grain porridges (do not underestimate the power of oatmeal), even pancakes (add butter to every layer plus syrup it's tasty), vegetarian tacos (add it to the filling while it cooks, I use lentils&brown rice plus tons of butter), goodness you would not believe the calories in flour tortillas. Try biscuits and gravy if you want to work on your culinary skills. Mashed potatoes are another great example. I know a lot of people who will add both half a stick of butter AND cream to their potatoes. They turn out tasting delicious!
Generally, when I add a lot of fat to a dish I try to balance it out with an acidic ingredient. This seems to aid digestion as well as improve the flavor immensely. It's not as obvious that you're eating 250 extra cal worth of butter when it is cooked with fresh tomato (in some dishes!!!)
Things I suggest avoiding adding extra fats to: .. low-carb foods, if you are looking to gain. Regularly combining an intake of carbohydrates with fats promotes weight gain over time.
I think the key is truly to balance carbs and fats. Usually the outcome is pretty intuitive. Just look for sauces, fillings, cooked grains and porridges things like that which will readily incorporate your additions.
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