Preheat the oven to 190°C, 170°C fan, 375°F, gas 5. Toss the cherry tomatoes and rosemary with 1 tablespoon olive oil in a small roasting tin. Roast for 25-30 minutes, until the skins have split and are beginning to blacken. Set aside to cool a little. Depending on whether you want to make full-sized muffins or mini muffins, grease 9 of the holes in a 12-hole muffin tin or two 12-hole mini muffin tins with the remaining half tablespoon of olive oil.
Combine the flour, Parmesan and gruyere in a mixing bowl. In a jug, whisk the milk, melted butter and egg together. Pour the wet ingredients into the dry, add the tomatoes and any juices and combine everything together quickly. Don’t worry about any lumps – they will ensure the muffins are light. Divide between the 9 large or 24 mini holes and slide into the oven.
Bake for 20 minutes if making large muffins, or about 15 minutes, if making mini muffins. They should be golden, risen and springy to the touch. Turn out and cool on wire racks. Eat plain or split and fill with gruyere cheese and ham.
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