I lower my carbs on fast days and increase protein and fat, e.g. By using eggs and fish, putting butter on green or coloured cooked veg or olive oil on salad. If you measure amounts carefully you won’t go OTT and fats help you feel full longer. I avoid all white carbs such as pasta, flour products and white potatoes. You can always have them next day if you want.
Frittata with roast or steamed veg is delicious and comes in at about 150-250 cals depending on egg size, whether you have one egg or two and how much fat you use. Very sustaining too.