Toast sunflower (or pumpkin) seeds in a large skillet over medium heat, stirring often, until fragrant and beginning to brown, 4 to 5 minutes. Transfer to a bowl and let cool.
Peel cucumbers. Cut in half lengthwise then slice ¼-inch thick.
Hull strawberries and cut into ¼-inch thick slices.
Trim romaine and cut into 1-inch pieces.
Mix spinach and romaine in a large bowl.
Mix spinach and romaine in a large bowl. Process vinegar, syrup, mustard, garlic powder, salt and pepper in a food processor fitted with a steel blade until combined. With the motor running, add oil in a stream and blend for 10 to 20 seconds.
Add the strawberries, cucumbers and the toasted seeds to the greens. Drizzle with dressing and toss to coat. Serve immediately.
