Welding thick steel plates in the range of 30-100 mm is crucial for the industrial sector of offshore platform construction. Laser welding of such thick sections could be a disruptive solution compared to the time- and material-consuming traditional arc-welding processes, which currently require multiple successive passes to complete the job. Although the power of today's available laser sources could, in principle, allow penetration up to 70 mm of steel, issues related to root formation discourage a single-pass welding approach for thicknesses greater than 25 mm. The use of a double-sided approach, which consists of two partial penetration passes, prevents root formation defects but, on the other hand, creates conditions for unwanted solidification cracks. By exploiting the dynamic beam shaping capabilities of the laser source and the single-mode characteristic of the laser beam, we can reduce the formation of solidification cracks to achieve a sound weld. Here, we use a double-sided, two-pass, laser welding method to achieve a defect-free butt joint of two 45 mm thick steel plates without the aid of a bevel. This result lays the foundation for a new laser welding process that can be further investigated to achieve sound welds for even thicker sections, potentially representing a paradigm shift for the joining of thick plates in the large construction industry.