Place bones and 1⁄4 cup kosher salt in a bowl and cover with cold water; chill and let soak to remove any blood, 1-2 days. Change the water three times during this period, adding another 1⁄4 cup kosher salt each time. Drain bones.
Place bones in a 6-qt. saucepan and pour enough water to come 1″ above bones. Place over medium-low heat, and bring to a gentle simmer; cook until marrow is hot in the center, about 25 minutes. Using tongs, transfer bones to a plate.
Meanwhile, heat 1⁄2 cup oil in an 8″ skillet over medium-high heat; add parsley, and cook until crisp, about 15 seconds. Using a slotted spoon, transfer to paper towels to drain; set aside and discard cooking oil. Heat a cast-iron grill pan over high heat, and brush challah bread with remaining oil; grill bread, turning once, until lightly charred with grill marks and toasted, about 2 minutes.
To serve, place warm marrow bones on a platter and sprinkle with fried parsley and fleur de sel; serve with challah toast on the side.