Retain the stems of mushrooms if they’re fresh. I used button mushrooms but use any kind you want or have. Quarter them lengthwise.
Put a large wide pan over high heat. Swirl the butter in the pan. Once the butter starts to melt, tip in your mushrooms. Get them in as even a layer as possible. Slight overlaps are fine. Let them sit and colour for 4-5 mins. Don’t move them around until they do. Don't salt at this stage.
Once the mushrooms have a nice char on the underside, toss them through for a few more mins to cook the other side. If there’s a lot of moisture in the pan just keep the heat on high and cook it out.
Add the garlic and toss until lightly coloured. Tip in your chopped spinach and season with salt and pepper. Keeping the heat on high, toss them about until they start to wilt down - no more than 2-3 mins. Decant into a small bowl and set aside.
Put the same pan back on the heat and add the tomatoes. Season with salt. Cover and cook for 5-7 mins until they soften. Set aside.
Whisk together the eggs, milk, Dijon mustard and salt in a wide shallow bowl.
Add a tiny knob of butter to a pan. Divide the cheese mixture between three sandwiches. Dip a sandwich gently in the egg mixture until the mixture is well absorbed but not so much that it falls apart in your hands. Two flips worked perfectly for me.
Add the sandwich to the pan and cook over a low heat, pressing down a few times to make sure the filling doesn’t fall out. Flip and cook until golden on both sides and the cheese is melted through.
Repeat with the rest of the sandwiches adding a knob of butter before each round.
Serve immediately with the mushroom and spinach topping, and the chunky tomatoes.