Cut the stale bread into 1x1 inch cubes. Rosemary herb bread was used, but any crusty bread will work. If you don’t have stale bread, you can toast the fresh bread cubes at 200°F for 30-40 minutes and then proceed with the rest of the recipe.
Combine the bread, grape tomatoes, cucumber, red onion, yellow onion, fresh basil, and parsley in a large bowl. Toss gently and let rest for a few minutes so that the juices from the tomatoes and cucumber start soaking into the bread.
In a mason jar or small bowl, whisk together the olive oil, apple cider vinegar, garlic, whole grain mustard, red pepper flakes, salt, and pepper until it has emulsified into a vinaigrette. Pour the dressing over the bread and vegetables and toss until completely combined.
The panzanella is best served right away at room temperature, or chilled up to 24 hours to allow the bread to soak up as much flavor as possible. Drizzle with balsamic vinegar and sprinkle Parmesan cheese as desired. Enjoy!