Whisk together mustard and lemon juice in a bowl. Add oil in a steady stream, whisking until combined thoroughly. Whisk in garlic. Season with salt and pepper.
Toss together greens and herbs in a bowl. Season with salt and pepper. Drizzle with 2 tablespoons vinaigrette and toss to coat. Vinaigrette can be refrigerated up to 1 week.
Mix It Up Farmers' markets are coming back to life, and finding baby leaves of kale, mustard, chard, spinach, arugula, and mache is easy. All you need is a mustardy vinaigrette to bring them into balance.