Heat 1 tablespoon oil in a large skillet over medium heat. Add chopped onion; cook, stirring occasionally, until softened, about 5 minutes.
Add 3 tablespoons tomato paste; cook, stirring constantly, until fragrant and deepened in color, 1 to 2 minutes.
Stir in chopped garlic, 1½ teaspoons Italian seasoning, ½ teaspoon salt and ¼ teaspoon crushed red pepper; cook, stirring often, until fragrant, about 30 seconds.
Add crushed tomatoes and rinsed beans; bring to a lively simmer over medium heat. Cook, stirring occasionally, until slightly thickened, about 3 minutes.
Fold in 5 ounces spinach; cook, stirring constantly, until bright green and just wilted, 2 to 3 minutes.
Remove from heat and stir in the remaining 2 tablespoons oil. Sprinkle with the remaining ⅛ teaspoon salt.
Divide the mixture among 4 bowls. Sprinkle with crumbled Parmesan. Garnish with additional crushed red pepper, if desired. Serve with toasted baguette slices.
