Finely chop the onion and bell pepper, then roughly chop the spinach. Set all aside for quick assembly.
Crack the eggs into a mixing bowl and whisk until the yolk and whites are fully combined and slightly frothy, about 20 seconds.
Place your cast iron skillet over medium-low heat and add a tablespoon of olive oil, letting it shimmer gently as it heats.
Add the chopped onion and bell pepper to the skillet. Sauté, stirring occasionally, until they are tender and fragrant, about 3-4 minutes. You should hear a gentle sizzle and see the vegetables start to caramelize around the edges.
Toss in the spinach and cook just until wilted, about 1 minute. The green will turn bright and shrink significantly, releasing a fresh aroma.
Pour the beaten eggs evenly over the sautéed vegetables. Let sit for about 10 seconds, then gently stir with a silicone spatula, folding the eggs over the vegetables. Keep the heat low to allow gentle cooking.
Stir occasionally, gently folding the eggs until they are softly set and slightly creamy, about 2-3 minutes. The eggs should be fluffy with some crispy edges forming around the vegetables.
If using cheese, sprinkle it over the eggs now and let it melt for a minute. Remove the skillet from heat once the eggs are just set and still slightly runny, as residual heat will finish cooking them.
Rest the scramble for a minute, then serve hot directly from the skillet. Garnish with additional herbs or cheese if desired, and enjoy the smoky, rustic flavors with crispy edges and tender vegetables.