In your Instant Pot, toast the cinnamon sticks, cloves, and allspice over medium heat until fragrant. This should take about 2-3 minutes.
Add halved onions and ginger slices to the pot. Continue toasting until the spices release their aroma and the onions and ginger become slightly charred.
Place the whole beef tenderloin in the Instant Pot. Pour 8 cups (1.9 liters) of water into the Instant Pot. Season with stevia, fish sauce, and salt. Stir the ingredients well.
Close the Instant Pot lid and set it to manual high pressure for 35 minutes. Allow the ingredients to simmer and infuse flavors. The beef will become tender, and the broth will absorb the aromatic essence of the spices and aromatics.
Once the broth is ready, carefully remove the whole beef tenderloin from the pot. Strain the broth to remove the spices, onions, and ginger. Allow the beef to cool slightly, then thinly slice it.
While the broth is simmering, prepare the rice noodles according to package instructions. Slice the chili peppers, red onions, and gather bean sprouts for serving.
In individual serving bowls, place a portion of cooked rice noodles. Arrange the thinly sliced beef on top of the noodles. Pour the hot broth over the beef and noodles, ensuring they are fully submerged. Serve the Pho hot, garnished with bean sprouts, sliced chili peppers, and julienne-cut red onions.