In a large bowl, thoroughly combine first 4 ingredients (ground beef through fennel seeds). Form mixture into 4 patties.
Brush mushroom caps with olive oil then season with salt and pepper.
Place beef patties in a large baking dish; bake for 5 to 10 minutes per side or until patties reach desired level of doneness.
Add portabella caps to baking dish; cook for 2 to 5 minutes per side or until tender.
To serve: Layer as follows: 1 mushroom cap, 1/2 cup spinach leaves, a beef patty and some sliced onion; top with a second mushroom cap. Serve warm.
Notes
SERVING SUGGESTION: Cucumber and Jicama Slaw – Cut cucumber and jicama into matchsticks. Place in a bowl then drizzle with some honey, lime juice, chili powder, salt and pepper; toss well and serve. NUTRITION: 552 Calories; 36g Fat (58.0% calories from fat); 48g Protein; 10g Carbohydrate; 3g Dietary Fiber; 285mg Cholesterol; 254mg Sodium. Exchanges: 0 Grain (Starch); 6 Lean Meat; ½ Vegetable; 1 Fat.