Bring a large pot of salted water to a boil. Add the pasta and cook according to the package directions until al dente. Drain and set aside.
While the pasta cooks, add 1 tablespoon of olive oil to a large skillet over medium-high heat. Add zucchini slices to the hot oil and cook for 1-2 minutes per side until lightly browned. Remove from the pan and set aside.
Add the sliced kielbasa to the skillet and cook until browned on both sides, about 3 minutes per side. Remove from pan.
Add the remaining 4 tablespoons of oil to the skillet. Whisk in the flour and cook for 1-2 minutes, stirring often. Add the minced garlic and ½ teaspoon of salt.
Turn the heat to low and slowly pour in milk while constantly whisking (to prevent lumps). Turn the heat back up to medium-high and bring to a slow boil, until the sauce has thickened. Remove the sauce from the heat and stir in sriracha sauce. Check the seasoning and add additional salt and pepper if needed.
Add the cooked zucchini, kielbasa, and pasta to the skillet. Stir until everything is well coated.
Transfer to a serving bowl or portion into individual bowls and garnish with some sliced green onions on top before serving warm.
Notes
You can start with 2 tablespoons of sriracha and add more to taste if you want the sauce to be more mild.