Heat your largest well-seasoned cast-iron or nonstick skillet over medium-high heat (if using cast iron, heat it for 2 to 3 minutes, until very hot, to prevent the tofu from sticking). Drizzle in 1 tablespoon of the oil and carefully add the tofu slices (the oil may spit, so stand back) in a single layer. Season generously with salt and pepper and cook, undisturbed, until the bottom is golden, 2 to 3 minutes. Flip the pieces and cook on the other side until the bottom is golden, another 2 to 3 minutes. Transfer to a plate.
Return the skillet to the stovetop and reduce the heat to medium. Add the remaining 1 tablespoon of oil, along with the onions, and season with 1/4 teaspoon of salt. Cook, tossing the onions, until soft and golden, 5 to 6 minutes.
Add the harissa and cook, stirring, until the paste starts to caramelize and stick to the pan, about 1 minute. Add the crushed tomatoes and water, and stir to loosen the sauce. Cover and cook until the flavors meld and the sauce thickens, about 10 minutes.
Season with the remaining 1/4 teaspoon of salt and a big pinch of black pepper. Taste to make sure the seasoning is right, and season with more salt and/or pepper, if desired. Add the tofu to the sauce and let warm through, about 2 minutes.
Serve from the skillet, topped with the herbs, if you like, and crusty bread or flatbread on the side.