Mix together all ingredients and season with salt and pepper. Set aside.
Combine vinegar, sugar, and salt in a container. Let sugar and salt dissolve.
Put onion slices in a colander and bring a few cups of water to a boil. Pour boiling water over the onions (this helps to dilute the flavor). Shake off excess water and transfer onions to the vinegar mixture.
If you can prepare the onions ahead of time, the flavor will be stronger. They are best if they can sit in the fridge for a few hours, but if you need them sooner squeeze the onions so they absorb more of the mixture faster.
In a small bowl, mix together lemon juice, freshly ground pepper, and mayo. On each slice of bread, spread butter on one side and mayo on the other.
Bring a large skillet to low heat and place two slices of bread butter-side down on the skillet. Place a slice of provolone on one slice of bread
Once the cheese is melted, top with tuna, a few slices of the hardboiled egg, pickled onions, and spinach. Top with the other slice of bread and remove from heat. Repeat the process with the other sandwich. Slice and serve.