What's a good breakfast I can take to work that is not too fattening, but is portable (in other words, easy)?

i have been eating muesli for breakfast every day for a few months. I take a little bottle of milk and i add fruit to the muesli, and it’s really good (and it’s some good roughage). However, I gained some weight which i feel may be due to this, as it’s my only dietary change. I used to eat a buttered roll every day, that was suitable (but a lot of carbs).

What else can i eat for breakfast at work that’s easy, filling and yet not too fattening?