In a medium bowl combine the cucumber, beans, cilantro, oil, vinegar, garlic, jalapeno, chili powder, and salt and pepper to taste. Set aside.
Place the tomatoes and onion slices on the lightly greased rack of a grill with a cover directly over medium heat; grill, uncovered, for 12 to 15 minutes or until lightly charred, turning onion slices once. Transfer vegetables to a cutting board; cool slightly and coarsely chop. Add chopped vegetables to the cucumber mixture; toss to combine.
Meanwhile, halve the French bread lengthwise. Cut each bread half crosswise into 3 pieces. Using a fork, hollow out the bread pieces slightly. Grill the bread pieces, cut sides down, about 1 minute or until toasted. Spoon the bean mixture into the bread pieces; sprinkle sandwiches with cheddar cheese. Place the sandwiches on the grill rack, filled sides up; close lid and grill for 1 to 2 minutes or until cheese is melted. Makes 6 servings.