This salad gets a boost from a finely ground pumpkin seeds in the dressing.
Heat a grill or grill pan until very hot. Brush 1 tablespoon olive oil on onion rounds; season with salt and pepper. Place on grill; cook until lightly charred and soft, about 6 minutes. Transfer to a plate; set aside.
Heat 1 tablespoon olive oil in medium skillet over medium-low heat. Add the pumpkin seeds and cook, stirring occasionally, until golden, about 18 minutes, being careful that the seeds don't jump from the skillet as they pop. Set skillet aside to cool.
Place half of the cooked pumpkin seeds in the bowl of a food processor; add a pinch of salt, and pulse until finely ground. With the machine running, slowly add the remaining 6 tablespoons olive oil. Add the lemon juice, and season with salt and pepper. Set the pumpkin vinaigrette aside.
When ready to serve, combine both lettuces and the grilled onion in a large bowl. Pour the reserved vinaigrette over the top, and toss the salad to combine.
Pumpkin seeds, also called pepitas, are available at gourmet supermarkets and Mexican markets.