Bring a large sauce pot of water to a boil. Add the udon and cook until just tender, 2 to 3 minutes. Drain and rinse with warm water, then return them to the pot and toss with 1 to 2 tablespoons oil to prevent sticking. Set aside.
While the water heats and the udon cooks, prepare the chicken and vegetables.

• Cut a small corner from the chicken packaging and drain off any excess liquid. Transfer to a plate; pat dry with a paper towel. Season generously with salt and pepper.

In a wok or large frying pan over medium-high heat, warm 2 tablespoons oil until hot but not smoking. Add the chicken and cook, stirring occasionally, until browned but not yet cooked through, 2 to 3 minutes. Transfer to a plate. Do not clean the pan.

• Trim the ends from the celery; thinly slice the celery on the diagonal.
• Peel and coarsely chop enough yellow onion to measure 1 cup.
• Remove the stems, ribs, and seeds from the red and green bell peppers; cut the peppers into 1-inch pieces.
• Trim the root ends from the scallions; thinly slice the scallions on the diagonal, keeping the white and green parts separate.

• In a small bowl, using a whisk or fork, mix together the arrowroot powder and chicken broth to form a slurry.

In the same pan used for the chicken, if dry, add 1 to 2 teaspoons oil. Warm over high heat until hot but not smoking. Stir in the celery, onion, bell peppers, and white parts of the scallions, season with salt and pepper, and cook until the vegetables just begin to brown, 1 to 2 minutes. Add the chicken and any accumulated juices. Stir the arrowroot slurry to recombine, then add the slurry, sesame stir-fry blend, and as much sambal oelek as you like. Bring to a boil, reduce to a simmer, and cook, stirring occasionally, until the chicken is cooked through and the sauce has thickened, 3 to 5 minutes. Season to taste with salt and pepper.
When the chicken and vegetables are almost done, chop the cilantro.

• Coarsely chop the cilantro.

Transfer the udon to individual bowls and spoon the chicken, vegetables, and sauce on top. Garnish with the cilantro and green parts of the scallions, sprinkle with the sesame seeds, and serve.