Prepare the Croutons: Preheat your oven to 400°F (200°C). Place the bread cubes on a large, rimmed baking sheet. In a small bowl, whisk together 2 tablespoons of olive oil, the garlic powder, and dried oregano. Drizzle over the bread and toss until evenly coated. Bake for 10-12 minutes, tossing halfway through, until crisp and golden brown. Let cool completely on the pan.
Make the Vinaigrette: While the croutons bake, combine all vinaigrette ingredients (3 tablespoons olive oil, vinegar, lemon juice, Dijon, minced garlic, fresh herbs, and pepper) in a small bowl or a glass jar. Whisk or shake vigorously until well combined and emulsified.
Macerate the Onion: Add the thinly sliced red onion to the vinaigrette and stir. Set aside for at least 10 minutes to allow the onion to soften and absorb the flavors.
Assemble the Base: On a large serving platter, arrange the cherry tomatoes, heirloom tomatoes, cucumber, and yellow bell pepper.
Layer the Salad: Scatter about half of the cooled croutons and all of the rinsed cannellini beans over the vegetables.
Dress and Serve: Give the vinaigrette a final stir. Using a fork, lift the onions from the dressing and sprinkle them over the salad, then drizzle the remaining vinaigrette evenly over everything. Top with the remaining croutons and fresh basil. For the best flavor, let the salad rest for 20 minutes at room temperature before serving.