My breakfast is 100g (~370kcal) of oats plus 300ml of sweetened soya milk (~125kcal), so nearly dead on 500kcal. It keeps me feeling nicely full until lunch. I eat that at least 3x a day (breakfast, post commute home snack, dessert). It gets me about 60g of protein, so about half what I supposedly need, along with a big dose of carbs.