Place the cubed salmon in a large bowl. Drizzle the olive oil over the salmon and sprinkle the cumin, chili powder, and coriander on top. Gently toss to coat. I actually prefer to use my bare hands to carefully toss and massage the oil and spices into he salmon. When done, cover the bowl and refrigerate for about an hour. During this time, you can make your chimichurri. (See below.)(Note: if you want to marinate the vegetables as well, double up on olive oil and spices and add them to the bowl to marinade. This is optional.) When the hour is up, remove the salmon from the fridge and let it come to room temperature. (Bringing your fish to room temp helps with an even cook.)In the meantime, cut and prep your vegetables. Line the skewers with salmon, zucchini, peppers, and onions. When it comes to the onions, pierce no more than one or two (if they're thin) at a time to avoid a mouthful of raw onion.
Preheat your grill to medium-high heat. Brush it well and make sure you're working with a very clean surface. (Gunk and debris will create an uneven surface and cause your fillets to stick and tear.) Next, lube up the grill rack with a paper towel dipped in canola oil. Working in strokes, repeat this five times, once every 15 seconds or so. This will create a nice and slick surface.
Place the kebobs on the grill and cook, turning occasionally, until the salmon is opaque with an internal temperature of 145° F throughout and vegetables are tender. This should take about 5-7 minutes.
When they're done, remove the kebobs and serve with chimichurri.