In a large pot or Dutch oven over medium heat, add the oil.

Reduce the heat to medium-low, add the onions, and cook, stirring occasionally, until they are golden-brown, about 15 minutes.

Add the garlic to the onions and cook, stirring occasionally, about 1 minute.

Add the chicken stock, the tomatoes and their juices, the saffron, the salt, and the black pepper to the onion mixture and bring to a boil, stirring frequently.

Reduce the heat and simmer the soup, about 15 minutes.

In a medium pot of salted, boiling water, cook the orzo for 7 minutes. It will finish cooking in the soup. Drain the water.

Add the cooked orzo and the heavy cream to the soup and simmer, stirring frequently, about 10 minutes.

While the soup is cooking, preheat a panini grill according to manufacturer's directions.

Place the 4 bread slices on a cutting board and lightly brush each, including the corners, with the melted butter.

Turn 2 of the slices over and sprinkle each with 2 ounces of the Gruyère cheese.

Place the 2 remaining bread slices, buttered-side up, on top of the cheese layers to create 2 sandwiches.

Place the cheese sandwiches on the heated panini grill and cook until they are just browned, about 5 minutes.

Place the sandwiches on the cutting board and let them rest for 1 minute. Cut the sandwiches into 1-inch-thick cubes.

Serve the soup topped with the grilled cheese croutons.