Is it a soup? Or is it a sandwich? It's both! These grilled panini sandwiches are full of caramelized onions and Gruyere cheese—it's French onion soup in sandwich form.
Add sliced onions and cook until caramelized about an hour and fifteen minutes, stirring frequently. (For detailed instructions, see yesterday's post on how to caramelize onions.)
Remove from heat and stir in salt and pepper to taste and thyme.
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