Adjust rack to top position and preheat oven to 425 degrees. Wash and dry produce.

Cut potatoes into ½-inch-thick wedges. Toss on a baking sheet with a drizzle of oil, oregano, salt, and pepper. Roast on top rack until browned and tender, 20-25 minutes.

While potatoes roast, peel and mince or grate garlic. Quarter lemon. Thinly slice tomato into rounds and season with salt and pepper.

In a small bowl, combine mayonnaise, mustard, ¼ tsp sugar (½ tsp for 4 servings), juice from one lemon wedge (two wedges for 4), and a pinch of garlic. Season with salt and pepper to taste.

Heat a drizzle of oil in a large pan over medium-high heat. Add spinach and remaining garlic; season with salt and pepper. Cook, stirring, until spinach is wilted, 2-3 minutes.

Turn off heat. Transfer spinach to a second small bowl; stir in cream cheese until combined.

Wipe out pan.

Spread half the sourdough slices with creamy spinach; top with even layers of feta, mozzarella, and tomato.

Spread remaining sourdough slices with Dijonnaise (save some for serving).

Close sandwiches.

Melt 1 TBSP butter in pan used for spinach over medium heat. Once hot, add sandwiches and push around in pan until melted butter has absorbed. (For 4 servings, work in batches or use a second pan, using 1 TBSP butter for each batch.) Cook until bread is golden brown and cheese is slightly melted, 4-6 minutes.

Add another 1 TBSP butter to pan, then flip sandwiches and push around again until melted butter has absorbed. Cook until bread is golden brown and cheese is fully melted, 4-6 minutes. TIP: If you have a heavy-bottomed pan, place on top of the sandwiches as they cook for a real panini experience!

Squeeze one lemon wedge over potatoes (two wedges for 4 servings).

Halve panini on a diagonal and divide between plates. Serve with potato wedges and remaining Dijonnaise on the side for dipping.