Combine ground fennel seeds, salt, ground cumin, and black pepper in a small bowl.
Heat 2 teaspoons oil in a large nonstick skillet over medium-high heat. Add 1/2 teaspoon spice mixture and fennel; sauté 15 minutes or until fennel is tender and lightly browned, stirring frequently. Transfer to a bowl; wipe pan clean with paper towels.
Heat remaining 2 teaspoons oil in pan; sprinkle steaks evenly with remaining spice mixture. Add steaks to pan; cook 4 minutes on each side or until desired degree of doneness. Remove from pan; let stand 10 minutes. Thinly slice steaks.
To prepare sandwiches, spread 2 teaspoons Homemade Mayonnaise onto each of 4 bread slices. Top each with one-fourth of beef, one-fourth of fennel, and 1/4 cup arugula. Top with remaining 4 bread slices.