In a large pot of boiling, salted water, cook the orzo until al dente, 8 to 10 minutes. Drain, reserving 1/2 cup of the pasta cooking water.
Meanwhile, in a large, deep skillet, heat the olive oil over medium heat. Add the onion, season with salt and pepper and cook until just golden, about 5 minutes. Stir in the half-and-half and 3/4 cup parmesan cheese and simmer, whisking often, until just thickened, 2 to 3 minutes.
Stir in the cooked orzo, reserved pasta water and the peas; season with salt and pepper. Cook until warmed through. Remove from the heat and fold in the spinach and scallions. Serve sprinkled with the remaining 3/4 cup parmesan cheese.