For the dressing: Combine the mustard, red wine vinegar, olive oil and Fresno pepper in a bowl and whisk until combined. Season with salt and pepper.
For the salad: In a large saucepan on medium-high heat, drizzle in a touch of olive oil and toss in the mortadella. Cook until slightly crispy, 5 to 7 minutes. Cook for another 3 minutes, then let cool on paper towels to absorb excess oil.
Place the green beans, cannellini beans, chickpeas, bell pepper, shallot and mixed greens in a large bowl. Toss the mixture with the vinaigrette and adjust the seasoning with salt and pepper. Top with the seared mortadella. Drizzle some olive oil over the top.