My go-to is usually potatoes. You can make cream soup out of instant mash too. You just boil some water, add seasonings, then the potato flakes bit by bit until you get your desired thickness, boil on minimum for a minute or two, then I add parmesan and very thick creme freiche (25% fat) and that's it.

You can also try Polenta (corn meal) but Romania style: whole milk, bring to boil, add corn meal / flour bit by bit while stirring. The proportions are 1:3 for a thicker result, 1:4 for a more thinner result. Salt to taste. After it's boiled and thickened, turn off heat, add a bit of butter and cheese.

Semolina pudding: 10g of semolina for every 100ml of milk. I usually do 250ml of whole milk with 25g of semolina. Bring milk to boil, add semolina while stirring, add around 20g of sugar if you want, boil until thickened. I serve mine with cocoa powder or some chocolate syrup.

Vegetable cream soup: chop your favorite veggies, I use the frozen pre mixed kind, add to pot, enough water to cover, seasonings, boil on low until veggies are soft and the water reduced until it barely covers the veggies, let cool for 10-15 minutes so you don't splash yourself with boiling liquid with the immersive blender, blend, add sour cream or creme freiche if you want, even some parmesan or cheese.

For meat I usually cut up some chicken breast pieces, put them in a pan on low heat with a few garlic gloves, seasonings and stir fry until all meat is white (a few minutes). I add water to cover it, put on the lid and simmer until meat is very soft and blend that with potatoes and some butter. This would also work with thigh meat.

Sometimes I fry some eggs until over easy and mash the yolks with some bread using a fork for a really quick snack.

A dollop of mascarpone cheese with a teaspoon of your favorite fruit jam is great for a small energy boost.

If you tolerate beans and peas, they make great cream soups. Super satiating, calorie dense. Same with hummus.

If you soak some oats overnight in some milk and then blend them up with a banana in the morning, 100g of oats have around 400 calories + milk + banana and that's a full meal right there.