Heat 2 tablespoons oil and butter in a large pot over medium-high heat.
Add sweet and red onions and stir to coat.
Meanwhile, place mushrooms caps, gill-side up, on the prepared pan.
Brush with 1 tablespoon oil and sprinkle with 1/4 teaspoon pepper.
Roast on the middle rack until tender, 10 to 15 minutes.
Remove from oven and preheat broiler to high.
Add garlic and thyme to the onions and cook, uncovered, for 2 minutes more.
Cook, stirring often, until most of the liquid has evaporated, about 1 minute.
Stir in broth and return to a simmer.
Reduce heat and simmer for 10 minutes.
Meanwhile, combine Gruyere, breadcrumbs, chives and the remaining 1 tablespoon oil in a small bowl.
Divide the mixture among the mushroom caps.
Broil the mushrooms until the filling is lightly browned, about 1 minute.
Divide the soup among 4 bowls and serve topped with a mushroom “toast.”
- Daily Values (DVs) are the recommended amounts of nutrients to consume each day.
(-) Information is not currently available for this nutrient.
