1/2 cup walnuts (2 oz), toasted (see Tips)
1/4 cup fine dry plain bread crumbs
1 lb Yukon Gold potatoes
2 medium fennel bulbs, trimmed, reserving fronds, bulbs halved and sliced lengthwise 1/4 inch thick
3/4 lb parsnips (3 medium), sliced diagonally 1/3 inch thick
1/2 lb carrots (3 medium), sliced diagonally 1/3 inch thick
1 large onion, halved and sliced
3/4 cup olive oil, divided
2/3 cup water
1/3 cup chopped dill
8 (17- by 12-inch) phyllo sheets, thawed if frozen