In a small bowl, combine the minced garlic, brown sugar, soy sauce, water, balsamic vinegar, corn starch, and Sriracha hot sauce. Whisk until well blended and set aside.
In a large skillet or wok, heat 1 tablespoon of sesame oil over medium-high heat. Once hot, add the flank steak strips. Cook for about 2-3 minutes, stirring occasionally, until the steak is browned and cooked through. Remove the steak from the skillet and set it aside.
In the same skillet, add the remaining 1/2 tablespoon of sesame oil. Add the sliced red, orange, and yellow bell peppers, along with the sliced red onion. Stir-fry for about 3-4 minutes until the vegetables are tender but still crisp.
Add the grated ginger and minced garlic to the skillet with the vegetables. Stir well and cook for another minute until fragrant.
Return the cooked steak to the skillet with the vegetables. Pour in the prepared sauce and toss everything together. Cook for an additional 2-3 minutes until the sauce thickens and coats the steak and vegetables.
Finally, stir in the fresh baby spinach and cook for just 1 minute until wilted. Remove from heat.
Serve the Sriracha pepper steak over a bed of rice, quinoa, or couscous. Enjoy your colorful and flavorful Sunday supper!