Heat the oven to 220C/200C fan/gas 7. Roughly chop any larger tomatoes and tip into a roasting tin along with any green vines, as these impart flavour, too. Using the side of your knife, lightly bash the garlic cloves and add to the tomatoes along with the lemon thyme. Drizzle over the oil and season well. Toss using your hands to combine. Roast for 25-30 mins until bursting and lightly charred. Carefully remove the garlic skins and mash into the tomatoes then cook for a further 15 mins. Set aside to cool slightly, then remove the thyme sprigs and discard.
Spread the mayonnaise over the outside of each of the four slices of bread, then spread the tomatoes over the other sides and top with the cheese. Sandwich the second half on top, with mayonnaise-side facing out, then put in a frying pan. Any leftover tomato mix will keep chilled in an airtight container for up to five days.
Put the pan over a medium-high heat, lay a baking tray or saucepan on top and weigh the toastie down with a couple of cans. You can also use a press if you have one. Fry for 4-5 mins until golden, then carefully turn, wiping the base of the tray or pan to remove any mayo, and press down again. Cook for a further 4-5 mins until golden, then cut in half and serve.
