Preheat broiler to high with rack in top third position (about 6 inches from heat source).
Heat 2 tablespoons oil in a large broiler-safe skillet over medium-high heat.
Add chicken pieces in an even layer; cook, undisturbed, until lightly browned on the bottom, 6 to 8 minutes.
Add minced shallots and garlic and ½ teaspoon each crushed red pepper, Italian seasoning and salt.
Cook, stirring often, until softened, about 2 minutes.
Stir in rinsed beans, 1 jar marinara, ¼ cup basil and 2 teaspoons vinegar, scraping up any browned bits; bring to a gentle simmer over medium heat.
Simmer, stirring occasionally, until the chicken is cooked through and the flavors meld, about 5 minutes.
Remove from heat and sprinkle with shredded mozzarella and the remaining ⅛ teaspoon salt.
Broil until the cheese is melted and browned in spots, about 6 minutes.
Sprinkle with the remaining ¼ cup basil and serve with the toasted baguette slices.
