Heat a pot of salted water until boiling. As it warms, rinse and cube the potatoes into small pieces.
Cook the potato cubes in boiling water for 10–15 minutes, until tender and a fork slides in easily; timing depends on cube dimensions and altitude.
Meanwhile, peel and chop the onion, then sauté it in 3 tbsp olive oil with a pinch of salt over low heat for about 5 minutes, until it becomes translucent.
Stir in the chopped bell peppers, jalapeño, red pepper flakes (if using), and 2 tsp Italian seasoning. Raise the heat to medium and cook for another 5 minutes. Add the vegan sausage, crumbling it into bite-sized bits, and cook until it starts to brown.
Drain the potatoes and leave them in a colander for about 5 minutes to shed extra moisture. Transfer them into the onion, pepper, and sausage mixture.
Mix thoroughly and continue to sauté for an additional 5 minutes. Adjust salt and black pepper to taste.