Begin by making the soup. This can be meal-prepped ahead of time, just be sure to heat it when you make your grilled cheese.
In a medium pot, add the olive oil and onion. Cook over medium heat until the onions soften, about 3-4 minutes.
Add the garlic and cook for 1 minute until fragrant. Add the tomato paste and toast it along with the onions and garlic for about 2-3 minutes until it develops a deeper red color.
Deglaze the pan with the crushed tomatoes and the chicken stock. Bring to a simmer. If your tomatoes are very tart, add a very small pinch of sugar.
Add the pepper flakes, basil, and Herbes de Provence. Simmer for 30-45 minutes until the flavors blend and the tomatoes are soft. You can optionally blend the soup using a hand blender or countertop blender for a smoother finish.
Right before making your grilled cheese, finish the soup with Greek yogurt for richness and a little tartness.
To make the grilled cheese, take two slices of bread and top them with equal amounts of cheese. Top the sandwiches with the other slice of bread. Butter the top slice and place it butter-side-down in a nonstick skillet over low heat.
Cook your grilled cheese for 6-8 minutes per side over low to ensure the cheese melts in the middle. Turning up the heat can quickly burn the outside of the bread and leave the cheese cold in the middle.
Once your grilled cheese is browned and crisp on both sides, remove it to a plate. Slice into 4 finger shapes vertically.
Serve the dippers alongside your soup for dunking.