Heat a large skillet or pot over low heat and add the 4 tablespoons butter and 1 tablespoon olive oil. Once melted, stir in the sliced onions and salt. Cook over low heat, stirring often, until the onions deeply caramelize, about 1 to 2 hours. Be sure to consistently check them and stir! While the onions are caramelizing, work on the dough.
In a bowl, stir together the water, yeast and honey. Let it sit for 10 to 15 minutes until foamy.
In the bowl of your stand mixer, add the flour, garlic powder, 1 tablespoon of salt, 1/2 cup of olive oil and the yeast mixture. Stir, using the dough hook, until the mixture comes together. Knead it on medium-low speed for 5 to 6 minutes. If it's super sticky, sprinkle in a bit of extra flour. You want it to be silky!
Put the dough in an oiled bowl and cover it. Place it in a warm spot and let it rise for 1 hour, or until it's doubled in size.
Once the dough has doubled, remove it from the bowl and divide it into 4 equal pieces. Roll and press the dough out until it’s about 6 to 8 inches in length and 4 or so inches in width. Line two baking sheets with parchment paper. Sprinkle the parchment paper liberally with flour so the dough doesn’t stick too much. Place the dough pieces on the baking sheets. Press the dough to make finger marks so it looks like traditional focaccia. Place the sheets in a warm spot and to rise again for 20 to 30 minutes.
After 30 minutes, heat your grill over medium heat – around 450 degrees F. Melt your remaining 4 tablespoons of butter and stir in the garlic cloves. Chop your favorite fresh herbs.
Flour a pizza peel or cutting board very well (or something where you can easily slide the dough from the peel to the grill). Transfer the dough pieces one at a time to the floured peel, moving quickly so they don’t stick. Press to make more finger marks and brush with olive oil.
Quickly transfer the dough by sliding it from the floured peel to the hot grill. Once it hits the grill, you can’t really move it without tearing the dough. Grill for 1 to 2 minutes, until the dough begins to bubble up and char. The dough can be flipped once it easily lifts from the grill. Flip the dough and grill for 1 to 2 minutes more, or again, until golden and puffed and brown. Remove the dough and place it on a baking sheet. Repeat with the remaining focaccia.
Brush the hot focaccia with the garlic butter. Sprinkle with the herbs. Sprinkle with the flaked sea salt. Cover with the caramelized onions. Slice and serve!