Roast the bell pepper. Preheat the oven to 400°F (204°C). Lightly brush a sheet pan with olive oil and place the bell pepper on top. Roast the pepper for 20 minutes, then using a pair of tongs, flip the pepper over and roast for another 20 minutes. Once the pepper is charred in spots, remove from the oven and cover it with a glass bowl or place in a resealable plastic bag. Steam the pepper for 15 minutes. Once steamed, roughly slice the pepper and remove the seeds.
While the pepper is roasting, prepare the soup in the Instant Pot. Select the Sauté setting on the Instant Pot. After a few minutes of heating up, add the 1 tablespoon olive oil, followed by the onion and carrots and season with 1/2 teaspoon of the salt and a few turns of black pepper. Cook the vegetables for 5 minutes, stirring occasionally.
Add the garlic, ginger, cumin and paprika, and cook for 1-2 minutes, coating the vegetables in the spices. Select the Cancel setting.
Add the harissa sauce, bay leaf, vegetable broth, tahini, coconut milk, and the remaining 1/2 teaspoon of salt. Stir to combine.
Select the Pressure Cook or Manual button at high pressure and set the cook time to 10 minutes. Make sure the sealing vent is set to Sealing.
Once the timer goes off, perform a quick pressure release by carefully turning the sealing vent from Sealing to Venting. Give the soup a stir and taste for seasonings.
Transfer half of the soup to a stand blender and add the sliced roasted bell peppers. Blend until the soup is creamy and smooth. Repeat with the remaining soup. Alternatively, you can use an immersion blender to purée the soup directly in the Instant Pot. Add a squeeze of fresh lemon and garnish the soup with scallions and cilantro if desired.