In a large bowl combine all the marinade ingredients together. Transfer 1/4 cup of the marinade to a large bowl and another 1/4 cup to another smaller bowl. To the first bowl with remaining marinade add the cubed beef and toss it well, making sure it's fully covered. Cover the bowl with plastic wrap and refrigerate for 30 minutes.: The air will be bright with the scent of garlic and lemon juice as you whisk the marinade, and you should notice the oil and tomato paste emulsify into a glossy mixture. This mixture coats the beef so the flavors can penetrate, which is why transferring two small portions matters, it gives you reserved glaze for brushing later. A common mistake is overmassing the beef in the bowl which can mash the cubes instead of coating them, so handle gently. Refrigerating lets the meat firm up and take in the flavors, and you will see the surface become slightly tacky when it has absorbed enough of the marinade.
Add the bell peppers and onions to the large bowl with 1/4 cup of reserved marinade and toss well. Set aside until ready to thread on skewers.: When you toss the vegetables, they should glisten with the reserved marinade, smelling of paprika and tomato. This light coating seasons them without making them soggy, which preserves their ability to char. If you skip tossing, the vegetables will cook flat and lack that layered caramelized flavor. Watch for the peppers to take on a faint sheen but remain firm, that indicates they are ready to go on the skewers alongside the beef .
Preheat your grill for 10 minutes on high heat. Lightly oil the grates of the gas grill.: As the grill heats, you will hear a steady hiss when you test the grates, and a properly preheated grill sears meat quickly, locking in juices. Lightly oiling prevents sticking and encourages those beautiful grill marks, which cue you visually that a crust is forming. Avoid heavy oiling which can cause flare ups, and a common error is not giving enough preheat time, resulting in pale, steamed kebabs rather than charred ones.
Thread the meat, onions, peppers on skewers, alternating until you run out of ingredients. You should get about 8 skewers.: Threading is where the dish comes alive visually, alternating colors and textures so each bite is balanced. You want the pieces snug but not squeezed, allowing heat to circulate for even cooking. A frequent slip is overcrowding the skewers, which prevents browning and can lengthen cook time. After threading, lift a completed skewer toward your nose; you should sense a marriage of savory spice and lemon, which tells you the flavors are well distributed.
Place the shish kebabs on the grill and cook for 8 to 10 minutes, turning occasionally making sure to grill on all sides. While the kebabs are grilling brush the skewers a couple times with the reserved 1/4 cup of marinade in the small bowl.: The sizzling should be confident, and you will hear intermittent pops as fat renders and touches the hot grates. Turning every few minutes encourages even browning, and brushing with reserved marinade builds layers of glossy flavor without burning. A mistake to avoid is constant flipping which prevents a stable crust, and another is brushing too early causing sugars to char; brush toward the middle and end of the cook for the best shine and depth. Look for edges caramelizing and juices beginning to bead on the surface as signals the kebabs are approaching done.
Transfer the kebabs to a serving platter and allow 5 minutes to rest before serving.: Resting is when the meat relaxes and juices redistribute, producing a juicier bite. During this brief pause the aroma will soften into a warm, inviting scent, and the surface will lose its initial fiery steam. The most common mistake is serving immediately which causes juices to spill out and leaves the meat dryer. Five minutes is enough for these sized cubes to settle, and when you cut into a piece the interior should be uniformly colored and tender to the touch.