Make the serrano ham croquette:
1. In a medium saucepan over medium heat, heat butter and oil. Add onion; season with salt and pepper. Cook, stirring occasionally, until onion is translucent, about 3 minutes. Add flour and cook, stirring, for 1 minute. Whisk in milk, and cook, continuing to whisk, for 3 minutes. Whisk in ham and nutmeg. Season again with salt and pepper. Spread mixture onto a baking sheet and let cool completely.
2. In a shallow dish, whisk eggs. Set dish aside. Using a tablespoon, scoop the cooled mixture and, using floured hands, shape into 2-inch rounds.
3. Coat each round in beaten egg, then in breadcrumbs. Transfer to a parchment-lined baking sheet and place in refrigerator. Allow to chill for about an hour before cooking.
4. In a large, heavy-bottomed stockpot fitted with deep-fry thermometer, heat 2½ inches of oil to 375 degrees. Working in batches to avoid crowding, fry the croquettes, flipping once, until dark golden brown, 1 to 2 minutes. Using a slotted spoon, transfer to paper towels to drain.
For the pan con tomate:
1. Using largest holes of a box grater, grate tomatoes into a large bowl. (Use a flat palm to move the tomatoes, so the flesh grates off while the skin remains intact.) Discard the skin and season tomato pulp with salt.
2. Set broiler to high. Place bread on a sheet tray and drizzle with olive oil. Broil until bread is crisp and starting to char around the edges.
3. Remove bread from oven and rub with garlic. Spoon tomato mixture onto bread. Drizzle with more olive oil and season with flaky sea salt. Set aside.
For the dijon hollandaise:
1. In a blender, combine egg yolks, mustard, lemon juice, and cayenne pepper. Cover and blend for about 15 seconds.
2. With the blender running on high speed, pour melted butter into egg yolk mixture in a thin stream. Hollandaise should thicken almost immediately. Keep warm until serving by placing the pitcher of the blender in warm water.
Place two slices pan con tomate on each plate. Top each slice with a croquette and a fried egg, then spoon hollandaise over top.