In a stainless skillet, heat the butter and olive oil over medium low heat. Add the shallots and sprinkle with a little kosher salt. Sauté the shallots on low for 15 to 20 minutes until they are tender and golden brown. Add the sugar and balsamic vinegar and cook until the vinegar has reduced. Taste and add more kosher salt if needed. Set aside.
Gremolata
In a small mixing bowl, combine the chopped parsley, lemon zest, minced garlic, and kosher salt. Add the lemon juice and olive oil and stir together. Set aside.
Roasted Bone Marrow
Preheat the oven to broil on high. Thinly slice the baguette on a bias and arrange on a baking sheet. Lightly drizzle both sides with a little olive oil and sprinkle with kosher salt. Broil the crostini for 3 to 5 minutes until golden brown. Flip the slices and broil the other side until golden brown. Keep an eye on them as they can burn easily. Set aside until service.
Preheat the oven to 450°F. Arrange the bones with the cut side facing up on baking sheets. If needed, use crumpled foil to anchor the bones so they don't tip over. Lightly drizzle the marrow with olive oil and sprinkle with kosher salt.
Roast the bones for 15 to 20 minutes until the bones are browned and the marrow is soft and lightly bubbling.
Arrange the bones on a serving platter. Spoon the warm caramelized shallots on top of the marrow. Top with the gremolata. Serve with the crostini on the side and enjoy!