You know that episode of Friends where Ross sticks a piece of gravy-soaked bread in the middle of his sandwich and calls it the moist maker? It sounds goofy, but it does hit on a key element of good sandwiches: They must be moist to be delicious. It's actually a pretty good life lesson in general.
This time, rather than using a single moist-maker, I'm going with a selection of three moderately moist ingredients—sautéed mushrooms, caramelized onions, and sun-dried tomato mayo—that together combine to make a synergistic Captain Planet-like explosion of moistness, all neatly contained in a crisp griddled crust.
First up: mushrooms sautéed until deeply browned. I used a mix of button mushrooms and portobello, sliced thin, but cremini (aka baby portobello) or shiitake would work as well. The key with sautéing mushrooms is to remember that cooking takes place in two distinct phases. When you first add the 'shrooms to the pan, they start expelling liquid out of their porous inner structure. It's only after this excess moisture has evaporated that they can begin to brown, which concentrates and adds complexity to their flavor.
Bear in mind: even though you're driving off water from the mushrooms' interior, the end results still taste moist due to the nice coating of olive oil the mushrooms get as they cook. A little garlic, lemon juice, and soy sauce amp up their flavor.
Next: a sun-dried tomato mayonnaise. Regular mayonnaise (you can use either a store-bought vegan mayonnaise or our homemade version) is nicely moist and tangy on its own. Processing some sun-dried tomatoes, some garlic, and a splash of lemon juice into them makes them more so, adding a sweet, jammy element as well.
Last ingredients: deeply caramelized onions. There's a rule with caramelized onions that is as unbreakable as the first law of thermodynamics and it goes like this: the longer you take to caramelize your onions, the better they will taste.
That said, your goal should always be to balance effort required with results achieved. Using my quick caramelization method, you can get onions that are about 80 percent as good as perfectly caramelized onions, in just a fraction of the time—as short as 15 minutes. In this case, I'm letting them go for about 30 minutes while I prep the rest of my ingredients.
Once all three elements are assembled, I slap them together and grill them on a panini press.
Pro-tip: don't smash your sandwich down all at once. This will cause your stiff crusts to break and compromise the structural integrity of your meal. Instead, close the lid gently and let the sandwiches cook for a few minutes to soften up the crust before pressing down more firmly.
The sandwiches that come out have a certain pizza-like quality to them, a result of the combo of tomatoes and onions with crisp crust. It's a good fix if you crave the intense sweet-and-savory flavor of a Totino's pizza roll, but in a more sophisticated and filling package.
And yes, they're moist.*
*I understand some folks are uncomfortable with the way that word sounds. I'm only sort of sorry I used it so much.