Season chicken tenderloins with salt and black pepper; pat into flour in a shallow bowl until lightly coated. Heat olive oil in a skillet over medium-high heat. Cook tenderloins, until browned, turning once, about 5 minutes per side. Remove from pan and set aside.

Reduce heat to medium-low; add garlic, onion, red bell pepper, and apple chicken sausage. Cook and stir until garlic and onion are fragrant and sausage is slightly browned, about 2 minutes. Pour in 1/2 cup chicken stock to deglaze the pan, scraping up browned bits. Add tarragon, sage, and garam masala; simmer for 2 more minutes.

Slice the chicken tenderloins into 1-inch pieces. Add chicken, remaining chicken stock, butternut squash, carrots, celery, and apple to pan; cover and bring to a boil. When stock is boiling, push ingredients to the edge of the skillet and add quinoa to the center of the pan. Cover and simmer over low heat until vegetables are soft and quinoa is cooked, about 20 minutes.

Mix in butter; stir the stew until slightly thickened, 3 to 5 minutes. Adjust seasoning to taste.