Cook bacon in a large skillet over medium heat until crisp; drain on paper towels, reserving 1/4 cup bacon drippings. Coarsely crumble bacon; set aside.
Brush tuna steaks with oil; sprinkle evenly with 1/4 teaspoon salt and 1/4 teaspoon pepper.
Grill, covered with grill lid, over medium-high heat (350° to 400°) 3 to 5 minutes on each side or until fish flakes with a fork. Cool fish and break into chunks.
Stir together garlic and butter; drizzle over bread cubes. Toss gently. Spread in a single layer on a lightly greased 15- x 10- inch jelly roll pan.
Bake bread cubes at 300° for 1 hour or until golden, stirring every 15 minutes. Sprinkle immediately with cheese, stirring to coat; cool. Set aside.
Process reserved 1/4 cup bacon drippings, remaining 1/4 teaspoon salt and 1/4 teaspoon pepper, cider vinegar, and next 3 ingredients in a blender or food processor until smooth, stopping to scrape down sides. Pour into a 1-cup glass measuring cup.
Microwave on high 30 to 45 seconds or until thoroughly heated.
Line salad plates with greens. Arrange tuna, bacon, beans, feta cheese, and croutons over lettuce. Drizzle with dressing; sprinkle with additional pepper. Serve immediately.