Caramelized onion mushroom melts
Caramelized onion mushroom melts are a rich, vegetarian grilled sandwich. Sweet golden onions and deeply browned mushrooms sit under melted provolone with a swipe of dijon mustard. The bread toasts crisp in butter. It takes 30 minutes, and most of that time is hands-off.
- 30 minTotal time
- 2Servings
- 600kcal / serving
- 25 gProtein
The science: why this works
Onions turn sweet slowly
Low heat and time break onion starches into sugars and brown them. A splash of water lifts stuck bits and keeps the onions from scorching. After about 15 minutes they turn deep golden and sweet.
Learn: Caramelizing onions →Mushrooms need space and heat
Mushrooms hold lots of water. In a hot pan, left alone for 3 minutes, that water evaporates and they brown. Stirring too soon steams them gray and rubbery, so give them plenty of room and some patience.
Learn: Browning mushrooms →Soy sauce adds umami
Soy sauce adds glutamates that boost the meaty depth of the mushrooms. Thyme adds an earthy, woodsy aroma. Together they make the filling taste rich and savory without any meat at all, which suits a vegetarian melt.
Learn: Building savory flavor →Caramelized onion mushroom melts
30 minutes · 2 servings · American · Sandwich
Ingredients
Makes 2 servings
Method
Tips for the best result
- Slice the onions thin and evenly.
- Do not crowd the mushrooms or they will steam.
- Cook the sandwich with a lid so the cheese melts before the bread darkens.
Swaps
- Swap the provolone for gruyere for a nutty, French onion feel.
- Swap the thyme for rosemary for a piney note.
- Swap the dijon mustard for whole grain mustard for more texture.
Storing and reheating
Store the onion and mushroom filling in the fridge for up to 4 days. Reheat to 165°F (74°C) before building. Make the sandwich fresh.
Caramelized onion mushroom melts: questions
How long does it take to caramelize onions?
Properly caramelized onions take about 15 to 30 minutes over medium-low heat. Rushing them on high heat burns them. This recipe uses 15 minutes for golden onions.
Why are my mushrooms soggy?
They were crowded or stirred too early. Use high heat and leave them alone for 3 minutes. Cook in batches if needed.
Can I make the filling ahead?
Yes, it keeps up to 4 days in the fridge. Reheat before using. It also freezes for 2 months.
