Stem, seed, and slice red and green bell peppers into thin 1/4" strips. Trim and thinly slice green onions on an angle, keeping white and green portions separate. Keep croutons in their shipping bag and crush into coarse chunks. Rinse chicken breasts and pat dry. On a separate cutting board, slice width-wise into 1/4" pieces. Season chicken with 1/4 tsp. salt and half the Cajun seasoning. Marinate 5 minutes.

Heat a large non-stick pan over medium-high heat. Add 2 tsp. olive oil and chicken to hot pan. Cook, stirring occasionally, 5-6 minutes, or until chicken is well-browned and reaches a minimum internal temperature of 165 degrees. Remove to a plate and return pan to heat (no need to wipe clean).

Add fettuccine to boiling water and cook 12 minutes, stirring often, or until al dente. Reserve 1/4 cup cooking water (to help thicken sauce later) and drain pasta in colander.

Heat pan used to cook chicken over medium heat and add 2 tsp. olive oil. Add red and green peppers and white parts of green onion to hot pan. Cook, stirring often, 6 minutes, or until softened. Remove to plate with chicken and return pan to heat (no need to wipe clean).

While pasta cooks, add evaporated milk and vegetable base to pan used in previous steps over medium heat. Bring to a simmer and cook 3-5 minutes, or until slightly thickened, stirring often to prevent skin from forming. Add half the remaining Cajun seasoning and 1/2 tsp. salt. Stir to combine, taste, and add additional Cajun seasoning if desired. Remove from burner and swirl in butter. If sauce is too thick, add reserved pasta water, 1 Tbsp. at a time, as needed. Add cooked pasta, chicken, and bell peppers to pan and toss to combine.

Twirl a mound of pasta on a plate and top with chicken. Garnish with remaining green onions and breadcrumbs.