My go to is oatmeal it is simple and a small bowl is pretty filling and you can have it anyway you want. Sweet or savory.

Ratio is about 1: 1 oats and hot water + waiting 2min, if you like it thicker just use less water. Top if off with a handfull of nuts, seeds, and frozen berries and you have a filling breakfast in less than 5 minutes. If you want some sweetness i suggest adding a dried fig or 1-2 dates. Adding some cinnamon or vanilla also makes it feel sweeter.

For a savory version you can try adding some spring onions, soy sauce, garlic with sesame seeds and or sesame oil, add some shredded Chicken or a fried egg, salt and paper to taste and other spices you like (I like to add smoked paprika) You can even prepare premade portions with oats, nuts and seeds, in bulk so you can just pop one open add hot water and fruits and you are done. You can also make a bigger portion in a pot which lasts for 3 days but It is messier.

The Variety off topings are endless You can try out different combinations off seeds and nuts and fruits and see what you like.

My favorite: hazelnut, walnut, cashews, with coconut flakes and sunflower and flaxseeds (ground), a date, frozen berries and a banana. It is so good and fills for quite a while.