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Grilled Portabella Sandwich with Walla Walla Onion Relish



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4 lb. Walla Walla onions, or other sweet onions, sliced 1/4" thick (about 3 qt.)
1/4 cup olive oil, divided, more as needed
1 lb. 12 oz. tomatoes, seeded, diced (about 1 qt.)
1/2 cup shredded fresh basil (about 3 oz.)
1/3 cup Jalapeno peppers, seeded, chopped (about 3 oz.)
1/4 cup white wine vinegar
1 tsp. salt, more as needed
1 tsp. pepper, more as needed
48 slices bread, such as sourdough
24 portabella mushroom caps (4 - 5 oz. each)
1 lb. 8 oz. Fontina, jack or other mild semi-soft cheese, in 1 oz. slices
24 fresh basil sprigs


  1. 1
    For Relish: Divide onion between 2 sheet pans. Drizzle 2 tbsp. olive oil over each pan; toss onion to coat. Bake onion at 400ºF , stirring occasionally, until tender and lightly browned, about 30 minutes; cool. Coarsely chop onion. Mix onion with remaining relish ingredients. Let stand at room temperature for about 30 minutes to marry flavors.
  2. 2
    Per Order: Grill or toast 2 slices of bread; reserve. Lightly score top of 1 Portabella mushroom cap in a diamond pattern; on the gill side; cut a shallow X in the center where stem was attached. Brush mushroom cap with olive oil; season with salt and pepper. Bake at 550ºF until tender and lightly browned, turning once, about 5 minutes. Put Portabella cap on 1 slice grilled bread. Top with 1/3 cup relish and 1 slice cheese. Partially cover with second slice of bread. Return to oven until cheese melts; about 1 minute. Garnish with basil sprig.

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cuisines North & South American, Other North American
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