Flavored Mayonnaise (July '00, p44), lettuce and tomato slices for serving
Preheat oven to 350°F. Spread walnuts on baking sheet and bake until lightly toasted, 8 to 10 minutes. Set aside to cool.
Meanwhile, in large saucepan fitted with steamer basket, bring 2 inches water to a boil over high heat. Add tempeh, cover and steam 20 minutes. Remove from steamer and set aside.
In food processor, pulse toasted walnuts to fine powder. Add half the steamed tempeh, mustard, tamari and umeboshi paste if using and puree until smooth. Transfer mixture to large bowl. Crumble remaining tempeh into very small pieces, add to mixture in bowl and mix well. Set aside.
In large skillet, heat 1 tablespoon oil over medium-high heat. Add onion and cook, stirring often, until softened, about 3 minutes. Add garlic, bell pepper, celery, rosemary, salt and pepper and cook, stirring often, 5 minutes. Add to tempeh mixture along with olives; mix well. Using 1/2 cup for each and packing firmly, form mixture into 8 round patties.
Prepare a hot charcoal fire or preheat gas grill to high. Place burgers on grill rack, brush with oil and cook until golden brown, 3 to 4 minutes per side. Serve hot on muffins with mayonnaise, lettuce and tomato slices.