Heat about a tablespoon of olive oil in a medium pan over medium heat.
Add sliced onion and sprinkle with a pinch of salt. Sauté onion until golden and caramelized, about 20 minutes. (If pan begins to dry out, just add a little splash of water and scrape up bits from the bottom.) Set aside until ready to use.
Meanwhile, combine olive oil and grated garlic clove in a small microwave safe cup. Microwave on high for 30-45 seconds until oil is fragrant. Set aside.
Drizzle a 9x13 inch sheet pan with about 2 tablespoons of olive oil. Preheat oven to 400 degrees.
In the bowl of an electric mixer, combine all ingredients and mix at high speed for about a minute until fully combined.
Scoop batter onto oiled pan, spreading dough out as much as possible. (The dough will be sticky.) Cover with a clean dish towl and let rise for an hour.
Uncover dough and poke with tips of fingers to make deep indentations. With a silicone basting brush, "paint" on prepared garlic oil. Drizzle dough liberally with plain olive oil.
Top dough with grapes, pressing them into the dough. Sprinkle with caramelized onion, rosemary and sea salt.
Bake for approximately 30-35 minutes or until the top develops a golden brown crust.
Remove from oven and let cool for 5 minutes, then remove from sheet pan with a spatula and transfer to a cutting board. Cut and serve warm.