Peel the onion and cut into -inch pieces.
Coarsely chop the cabbage.
Cut the zucchini into -inch pieces.
Cut the tomatoes in half. I personally don't care for warm tomatoes, so I skip these.
Pat the sausages dry with a paper towel; prick in a few places with a fork, if desired.
In a large frying pan over medium-high heat, warm 1 to 2 teaspoons oil until hot but not smoking. Add the sausages and cook, turning once, until browned but not yet cooked through, 3 to 4 minutes per side. Transfer to a cutting board to rest. Do not clean the pan.
In the same pan used for the sausages, stir in the onion, season with salt and pepper, and cook over medium heat until starting to soften, 1 to 2 minutes. Add the cabbage and zucchini and cook, stirring occasionally, until the cabbage begins to wilt and the zucchini starts to soften, 2 to 3 minutes. Stir in the tomatoes, season with salt and pepper, and cook until the tomatoes start to soften, 1 to 2 minutes.
While the vegetables cook, slice the sausages and scallions.
Cut the sausages on the diagonal into -inch-thick slices.
Trim the root ends from the scallions; thinly slice the scallions for garnish.
To the pan with the vegetables, stir in the sausages and cook until the sausages are cooked through, 1 to 2 minutes.
Transfer the sausages and vegetables to individual bowls or plates. Garnish with the scallions and as much green harissa as you like and serve.
