Any meat, fish, eggs are fine.
Omelettes/scrambled egg with ham, cooked chicken, cheese and/or bacon and maybe add mushrooms and spring onion. Eaten hot or cold.
Tuna in a sugar free mayo with green salad.
Ready cooked bacon can be reheated in a microwave for a few seconds, have with mushrooms and avocado.
Easy snacks such as cheese, cooked meat, pork scratchings, roasted chicken legs.
Greek yogurt with a few berries - blueberries travel well and keep for days in the fridge. Add some sweetener such as truvia or erythritol if wanted.