Combine the olive oil, balsamic vinegar, thyme, sea salt, garlic, and grape tomatoes in a small bowl.
Mix everything together and set it aside absorb the flavor.
In a pan with your cooking oil over medium-high heat, sauté the onion until it softens and turns light brown, about 3 – 5 minutes.
Add the garlic and cook for another couple minutes until fragrant.
Then, add the kale and continue to sauté until it is bright green, just another minute or two.
In a bowl, crack your eggs and beat them.
Whisk in the Gruyere, salt and pepper.
Pour the egg mixture in the skillet over medium heat.
Use a spoon to evenly stir the onion and kale to bring it up form the bottom a bit.
Scoop the tomatoes out on top of the frittata, leaving behind the marinade.
Save the marinade for later (trust me on this).
Cook over medium heat for 5 minutes, or until the edges begin to puff up. If the bottom seems like it is cooking too quickly, you can turn down the heat.
Once the edges are puffy, pop it under the broiler for another minute or two. The center should set and the top will get golden brown. Keep an eye on this as it progresses – it can happen really fast depending on your broiler.
OPTIONAL: Drizzle some of the remaining balsamic marinade over top of our frittata. It’s a nice little something something extra that makes you smile.