Bring a small sauce pot of water to a boil and fill a small bowl with ice water. Carefully lower the egg into the boiling water and cook for 10 to 12 minutes for hard-cooked. Transfer the egg to the ice water to cool. Carefully peel the egg.
While the egg cooks and cools, prepare the udon noodles.

Bring a medium sauce pot of water to a boil. Add the udon and cook, stirring occasionally to prevent sticking, until just tender, 2 to 3 minutes. Drain and rinse with cold water. Set aside.
While the water heats and the udon cooks, start the curry.

Prep and cook instructions are identical for both chicken options.

• Pat the chicken dry with a paper towel; cut the chicken into 1-inch pieces. Transfer the chicken to a medium bowl; season generously with salt and pepper, sprinkle with the chickpea flour and paprika, and toss to coat.
• Peel and thinly slice enough onion to measure 1¼ cups [2½ cups]; set aside ¼ cup [½ cup] sliced onion for garnish.

In a wok or large frying pan over medium-high heat, warm 1 to 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. Add 1 cup [2 cups] onion, season with salt and pepper, and cook, stirring occasionally, until starting to soften, 2 to 3 minutes. Stir in ¼ cup water, scraping up any browned bits from the bottom of the pan. Add the coconut curry base and spinach (the spinach in batches if needed) and cook, stirring occasionally, until the chicken is cooked through and the spinach is wilted, 2 to 3 minutes. Remove from the heat and season to taste with salt and pepper.
While the curry cooks, prepare the garnishes.

• Thinly slice the egg crosswise.
• Remove any coarse stems from the cilantro sprigs.

Transfer the udon to individual bowls and top with the chicken curry. Garnish with the egg, cilantro, and remaining sliced onion and serve.