Drop cream cheese packets in a glass of warm water to soften. Wash and dry produce.

Thinly slice tomato into rounds. Thinly slice mozzarella into rounds.

In a small bowl, whisk together cream cheese and pesto.

Arrange half the sourdough slices on a clean work surface. Spread with creamy pesto. Layer with tomato and mozzarella; season with pepper. Close sandwiches.

Melt 1 TBSP butter in a large, preferably nonstick, pan over medium heat. (For 4 servings, work in batches or use a second large pan, using 1 TBSP butter for each batch.) Add sandwiches and push around in pan until butter has absorbed. Cook until bread is golden brown and cheese slightly melts, 4-6 minutes.

Add another 1 TBSP butter to pan, then flip sandwiches and push around again until butter has absorbed. Cook until bread is golden brown and cheese fully melts, 4-6 minutes more. TIP: If you have a heavy-bottomed pan, place on top of the sandwiches as they cook for a real panini experience!

While sandwiches toast, in a second small bowl, combine half the vinegar, half the mustard, 2 TBSP olive oil, ¼ tsp sugar, a pinch of salt, and pepper. (For 4 servings, use all the vinegar, all the mustard, 4 TBSP olive oil, and ½ tsp sugar.) Whisk to combine.

In a large bowl, toss arugula with as much vinaigrette as you like.

Halve panini on a diagonal. Divide between plates and serve with salad on the side.