Dressing 1 cup mayonnaise

1/3 cup shopped fresh basil

2 tablespoons Dijon mustard

1 teaspoon fresh lemon juice

1/3 cup olive oil

1 tablespoon minced garlic

Everything else 1 1/2 cups mesquite wood chips -- soaked in cold

water 1 hour (optional) 6 to 5-inch-diameter portobello mushrooms -- =A5 stems

removed 6 to 4-inch-diameter whole-grain hamburger -- =A5=

split buns 6 large romaine lettuce leaves

6 large tomato slices

Mix first 4 ingredients in small bowl. Season with salt and pepper. Whisk olive oil and garlic in another small bowl. Prepare barbecue (medium-high heat). When coals turn white, drain chips, if using, and scatter over coals. When wood chips begin to smoke, brush mushroom caps on both sides with garlic oil. Season with salt and pepper. Grill mushrooms until tender and golden brown, about 4 minutes per side. Transfer to platter; cover with foil to keep warm. Grill cut side of hamburger buns until light golden, about 2 minutes. Place bottom half of hamburger bun on each plate. Top each with 1 mushroom, then 1 lettuce leaf and 1 tomato slice. Spoon some basil-mustard sauce over tomato and top with bun. Pass remaining basil-mustard sauce separately. 6 Servings=20 Bon App=E9tit July 1995=20 Brushed with garlic oil, grilled over the fire and stacked onto crusty buns with plenty of burger trimmings, portobellos are this season's snazziest alternative sandwich filling.