Cheese and onion sandwich with mayo

Oatmeal Bread [Wheat Flour (Wheat Flour, Calcium Carbonate, Iron, Niacin, Thiamin), Water, Oatmeal, Wheat Bran, Yeast, Salt, Wheat Gluten, Spirit Vinegar, Emulsifiers: Mono-and Diglycerides of Fatty Acids, Mono - and Diacetyl Tartaric Acid Esters of Mono-and Diglycerides of Fatty Acids; Vegetable Oils (Rapeseed Oil, Palm Oil), Malted Barley Flour, Flour Treatment Agent: Ascorbic Acid; Palm Fat, Wheat Starch], MAYONNAISE (25%) [Water, Rapeseed Oil, Cornflour, Pasteurised Salted Egg Yolk (Egg Yolk, Salt), Spirit Vinegar, Sugar, Dijon Mustard (Water, Mustard Seeds, Spirit Vinegar, Salt), White Wine Vineqar, Salt), MEDIUM MATURE CHEDDAR CHEESE (18%) (Milk), RED LEICESTER CHEESE (9%) (Cheese (Milk), Colour: Annatto), RED ONION (4.5%).