Place butter and oil in a 23 cm (9 inch) non stick frypan over low heat and cook until butter has melted; add onions to pan.
Cook onions, stirring occasionally, for 10-15 minutes or until onions are golden brown, soft and caramelized.
Place eggs, cream and pepper in a bowl and whisk to combine; pour egg mixture over onions in frypan and sprinkle with cheese and thyme and cook frittata for 5-6 minutes or until it is almost set.
To finish cooking, place frittata under a preheated hot grill for 1 minute; cut into wedges and serve with hot buttered toast or with a spicy chutney.
Pumpkin Frittata - Add 1/2 cup mashed pumpkin or sweet potato when whisking eggs.
Blue Cheese and Potato Frittata - When onions have cooked in the pan, sprinkle 1 cup cooked cubed potato over them, pour in egg mixture and sprinkle blue cheese instead of cheddar over eggs.
Antipasto Frittata - After adding eggs to the pan, top them with char grilled antipasto vegetables such as capsicum (red or green pepper), eggplant (aubergine) and zucchini (courgette), sprinkle frittata with cheddar and continue with steps three and four.