Caramelize the onions. Melt butter in a pan over medium-low heat. Add onions and let them sit for about 20 minutes as they begin to caramelize. Stir every 10 minutes or so, ensuring they don't turn too dark. As the fond starts to form on the bottom of the pan, add a ½ tablespoon of sherry and deglaze the pan (scrape as much of the dark bits from the bottom as possible). As the onions continue to cook, stir them every 10 minutes until they are done. Once they are a nice golden brown color, add the additional ½ tablespoon of sherry and deglaze again. Remove from heat and let cool slightly.
Mix the dip base. While the onions are cooking, mix the greek yogurt and cream cheese in a small mixing bowl. Add the garlic powder, pepper, salt and Worcestershire sauce; mix well until smooth and creamy. Refrigerate until the onions are done cooking.
Make the dip. Once the onions have cooled, remove them from the pan and place them on a cutting board, reserving a tablespoon or two for topping. Chop the onions and then add them to the yogurt mixture; stir until incorporated. Return to refrigerator while the bread is toasting.
Make the compound butter. In a small bowl mix the butter, herbs and sea salt.
Toast the bread. Slice the baguette on an angle to form longer pieces, about an inch thick. Butter one side of each baguette slice. Heat a large non-stick skillet over medium heat. Place bread slices in skillet, buttered side down and toast for 2-3 minutes until golden brown. Repeat with additional slices until all baguette slices are toasted.
Serve. When ready to serve, top dip with reserved caramelized onions.