For the baguette cubes: Preheat the oven to 400 degrees F.
In a large bowl, add the baguette cubes and drizzle over the olive oil. Toss until the bread is coated. Sprinkle over a pinch of salt and a few grinds of pepper, then toss again. Pour onto a nonstick baking sheet and toast, tossing every 3 to 4 minutes, until golden on all sides, 12 to 14 minutes. Allow the cubes to cool slightly before tossing back into the large bowl.
For the vinaigrette: In a medium bowl, combine the mustard and lemon zest and juice. Slowly whisk in the olive oil. Season with salt and pepper. Add the jalapenos and onions and allow them to soak in the vinaigrette for 10 minutes.
For the salad: To the large bowl of toasted baguette cubes, add the baby spinach, black olives and tomatoes. Toss gently. Pour over the vinaigrette and toss until combined. Serve at room temperature.