Preheat oven to 375ºF. To make sauce, heat oil in a large saucepan over medium heat. Add onion and jalapeño and cook until softened, but not browned. Add garlic and cook a minute longer. Stir in flour and cook a minute more, then gradually whisk in chicken broth. Cook, stirring occasionally, until sauce simmers and thickens. Remove from heat and slowly whisk in the sour cream. Season to taste with salt.

Preheat grill (or grill pan) on medium heat. Brush zucchini and chicken breasts with oil and season with salt and pepper. Grill zucchini for 6 minutes, until grill-marked and crisp-tender, and cook chicken until no longer pink inside (or registers 160º F - it’ll rise another 5º as it rests), 8-12 minutes, flipping once. Chop chicken and zucchini into bite-sized pieces.

Spread 1 cup of the sauce in bottom of a 9x13” baking pan.

Lay the tortillas on a flat work surface and divide the chicken and zucchini equally among them, positioning the filling in a line along the centre. Set aside 1 cup of cheese for topping, then divide the remaining cheese among tortillas. Drizzle 12 tbsp of the sauce on top of the filling (2 tbsp per tortilla), then fold in the edges and roll them up. Nestle them into the baking pan, seam side down, as you roll them. Pour the rest of the sauce over top of the enchiladas, sprinkle with reserved cheese, and then bake for about 30 minutes, until sauce is bubbling and filling is heated through.