Wash the romaine lettuce under cold water, roughly chop it into bite-sized pieces, and place it in a large salad bowl. Halve the cherry tomatoes and finely dice the red onion. Set aside.
In a dry skillet over medium heat, add the walnut halves and toast them for about 5-7 minutes, stirring occasionally until golden brown and fragrant. Remove from heat and allow to cool.
In a small bowl, whisk together the balsamic vinegar, olive oil, Dijon mustard, salt, and pepper. Adjust seasoning as needed.
In the large salad bowl, add the halved cherry tomatoes, diced red onion, toasted walnuts, and crumbled blue cheese. Drizzle the dressing over the top.
Gently toss the salad to combine all ingredients evenly and coat with dressing. Serve immediately.
